Eliminate Per-Frame Allocation in Rendering Loop

We resolved **Issue 4.3 (Severe Per-Frame Allocation in Rendering
Loop)** by removing the intermediate `MeshData` structure and storing
`macroquad::prelude::Mesh` directly in
[`DrawCommand::Mesh`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs).

[`DrawCommand`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs)
- Dropped `Clone` and `Debug` derives from
  [`Turtle`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs#L58)
  and
  [`DrawCommand`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs#L351),
  accommodating Macroquad's `Mesh` which does not implement these
  traits.
- Changed
  [`DrawCommand::Mesh`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs#L353)
  to the tuple variant `DrawCommand::Mesh(macroquad::prelude::Mesh)`.
- Deleted `struct MeshData` and its `to_mesh` method.

[`tessellation.rs`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs)
- Renamed `build_mesh_data` to
  [`build_mesh`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L35)
  returning `macroquad::prelude::Mesh { vertices, indices, texture: None
  }` directly.
- Updated
  [`tessellate_polygon`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L71),
  [`tessellate_multi_contour`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L116),
  [`tessellate_stroke`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L214),
  [`tessellate_circle`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L259),
  and
  [`tessellate_arc`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/tessellation.rs#L308)
  to return `Result<Mesh, Box<dyn std::error::Error>>`.

[`drawing.rs`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/drawing.rs)
- In the main drawing loop
  ([`drawing.rs:36-38`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/drawing.rs#L36-L38)):
  ```rust
  DrawCommand::Mesh(mesh) => {
  draw_mesh(mesh);
  }
  ```
  Now borrows `&Mesh` directly from the command vector with **0 heap
  allocations and 0 vector clones per frame**.
- Updated fill preview ([line
  228](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/drawing.rs#L228)),
  tween arc/center indicators ([lines 309,
  329](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/drawing.rs#L309)),
  and turtle shape drawing ([line
  349](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/drawing.rs#L349))
  to pass `&mesh` directly to `draw_mesh(&mesh)` without vector cloning.

[`execution.rs`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/execution.rs)
- Updated
  [`commands.push(DrawCommand::Mesh(mesh))`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/execution.rs#L135)
  and
  [`tessellate_command`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/execution.rs#L337).
- Updated
  [`test_forward_left_forward`](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/execution.rs#L487)
  to instantiate `mut state` directly without dummy cloning.

---

```bash
cargo test --lib
```
Output:
```
running 10 tests
test circle_geometry::tests::test_circle_left_geometry ... ok
test circle_geometry::tests::test_circle_right_geometry ... ok
test general::angle::tests::degrees_to_radians_roundtrip ... ok
test general::angle::tests::from_integer ... ok
test execution::tests::test_forward_left_forward ... ok
test general::angle::tests::negation ... ok
test tweening::tests::test_animated_mode_pops_to_current_tween ... ok
test tweening::tests::test_instant_mode_drains_queue ... ok
test
tweening::tests::test_instant_mode_respects_batch_limit_and_retains_pending
... ok
test tweening::tests::test_streaming_append_commands_does_not_accumulate
... ok

test result: ok. 10 passed; 0 failed; 0 ignored; 0 measured; 0 filtered
out; finished in 0.00s
```

All targets verified with `cargo check --all-targets` and `cargo clippy
--lib`.

turtle-lib/src/drawing.rs

@@ -33,8 +33,8 @@ pub(crate) fn render_world_with_tweens(world: &TurtleWorld, zoom_level: f32) {
     for turtle in &world.turtles {
         for cmd in &turtle.commands {
             match cmd {
-                DrawCommand::Mesh { data } => {
+                DrawCommand::Mesh(mesh) => {
-                    draw_mesh(&data.to_mesh());
+                    draw_mesh(mesh);
                 }
                 DrawCommand::Text {
                     text,
@@ -224,8 +224,8 @@ pub(crate) fn render_world_with_tweens(world: &TurtleWorld, zoom_level: f32) {
                     &all_contours,
                     fill_state.fill_color,
                 ) {
-                    Ok(mesh_data) => {
+                    Ok(mesh) => {
-                        draw_mesh(&mesh_data.to_mesh());
+                        draw_mesh(&mesh);
                     }
                     Err(e) => {
                         tracing::error!("Failed to tessellate fill preview: {:?}", e);
@@ -304,9 +304,9 @@ fn draw_tween_arc(
     );
 
     // Draw center using Lyon tessellation this helps visualizing what is done.
-    if let Ok(mesh_data) = crate::tessellation::tessellate_circle(geom.center, 5.0, GRAY, true, 1.0)
+    if let Ok(mesh) = crate::tessellation::tessellate_circle(geom.center, 5.0, GRAY, true, 1.0)
     {
-        draw_mesh(&mesh_data.to_mesh());
+        draw_mesh(&mesh);
     }
 
     // Calculate how much of the arc we've traveled based on tween progress
@@ -316,7 +316,7 @@ fn draw_tween_arc(
     let progress = CubicInOut.tween(1.0, t as f32); // tween from 0 to 1
 
     // Use Lyon to tessellate and draw the partial arc
-    if let Ok(mesh_data) = crate::tessellation::tessellate_arc(
+    if let Ok(mesh) = crate::tessellation::tessellate_arc(
         geom.center,
         radius,
         geom.start_angle_from_center.to_degrees(),
@@ -326,7 +326,7 @@ fn draw_tween_arc(
         ((steps as f32 * progress).ceil() as usize).max(1),
         direction,
     ) {
-        draw_mesh(&mesh_data.to_mesh());
+        draw_mesh(&mesh);
     }
 }
 
@@ -343,10 +343,10 @@ pub(crate) fn draw_turtle(turtle_params: &TurtleParams) {
                 .collect();
 
             // Use Lyon for turtle shape too
-            if let Ok(mesh_data) =
+            if let Ok(mesh) =
                 tessellation::tessellate_polygon(&absolute_vertices, Color::new(0.0, 0.5, 1.0, 1.0))
             {
-                draw_mesh(&mesh_data.to_mesh());
+                draw_mesh(&mesh);
             } else {
                 // Fallback to simple triangle fan if Lyon fails
                 let first = absolute_vertices[0];

turtle-lib/src/execution.rs

@@ -123,7 +123,7 @@ pub(crate) fn execute_command_side_effects(
                 }
 
                 if !fill_state.contours.is_empty() {
-                    if let Ok(mesh_data) = tessellation::tessellate_multi_contour(
+                    if let Ok(mesh) = tessellation::tessellate_multi_contour(
                         &fill_state.contours,
                         fill_state.fill_color,
                     ) {
@@ -132,7 +132,7 @@ pub(crate) fn execute_command_side_effects(
                             contours = fill_state.contours.len(),
                             "Successfully created fill mesh - persisting to commands"
                         );
-                        commands.push(DrawCommand::Mesh { data: mesh_data });
+                        commands.push(DrawCommand::Mesh(mesh));
                         #[cfg(feature = "svg")]
                         svg_log.push(crate::state::SvgRecord::Fill {
                             contours: fill_state.contours,
@@ -326,7 +326,7 @@ pub(crate) fn tessellate_command(
 
     match command {
         TurtleCommand::Move(_) | TurtleCommand::Goto(_) => {
-            let mesh_data = tessellation::tessellate_stroke(
+            let mesh = tessellation::tessellate_stroke(
                 &[start.position, end_position],
                 start.color,
                 start.pen_width,
@@ -334,7 +334,7 @@ pub(crate) fn tessellate_command(
             )
             .ok()?;
 
-            Some(DrawCommand::Mesh { data: mesh_data })
+            Some(DrawCommand::Mesh(mesh))
         }
 
         TurtleCommand::Circle {
@@ -350,7 +350,7 @@ pub(crate) fn tessellate_command(
                 *radius,
                 *direction,
             );
-            let mesh_data = tessellation::tessellate_arc(
+            let mesh = tessellation::tessellate_arc(
                 geom.center,
                 *radius,
                 geom.start_angle_from_center.to_degrees(),
@@ -362,7 +362,7 @@ pub(crate) fn tessellate_command(
             )
             .ok()?;
 
-            Some(DrawCommand::Mesh { data: mesh_data })
+            Some(DrawCommand::Mesh(mesh))
         }
 
         // `produces_drawing()` guards entry — this arm is only reachable if
@@ -484,7 +484,7 @@ mod tests {
         // the turtle ends up at (100, -50) from initial position (0, 0)
         use crate::state::TurtleParams;
 
-        let state = Turtle {
+        let mut state = Turtle {
             turtle_id: 0,
             params: TurtleParams {
                 position: vec2(0.0, 0.0),
@@ -503,21 +503,6 @@ mod tests {
             tween_controller: TweenController::default(),
         };
 
-        // We'll use a dummy world but won't actually call drawing commands
-        let world = TurtleWorld {
-            turtles: vec![state.clone()],
-            camera: macroquad::camera::Camera2D {
-                zoom: vec2(1.0, 1.0),
-                target: vec2(0.0, 0.0),
-                offset: vec2(0.0, 0.0),
-                rotation: 0.0,
-                render_target: None,
-                viewport: None,
-            },
-            background_color: Color::new(1.0, 1.0, 1.0, 1.0),
-        };
-        let mut state = world.turtles[0].clone();
-
         // Initial state: position (0, 0), heading 0 (east)
         assert_eq!(state.params.position.x, 0.0);
         assert_eq!(state.params.position.y, 0.0);

turtle-lib/src/state.rs

@@ -55,7 +55,6 @@ impl Default for TurtleParams {
 }
 
 /// State of a single turtle
-#[derive(Clone, Debug)]
 pub(crate) struct Turtle {
     pub(crate) turtle_id: usize,
     pub(crate) params: TurtleParams,
@@ -349,30 +348,11 @@ pub(crate) enum SvgRecord {
     },
 }
 
-/// Cached mesh data that can be cloned and converted to Mesh when needed
-#[derive(Clone, Debug)]
-pub(crate) struct MeshData {
-    pub(crate) vertices: Vec<macroquad::prelude::Vertex>,
-    pub(crate) indices: Vec<u16>,
-}
-
-impl MeshData {
-    #[must_use]
-    pub fn to_mesh(&self) -> macroquad::prelude::Mesh {
-        macroquad::prelude::Mesh {
-            vertices: self.vertices.clone(),
-            indices: self.indices.clone(),
-            texture: None,
-        }
-    }
-}
-
 /// Drawable elements in the world.
 /// All drawing is done via Lyon-tessellated meshes for consistency and quality.
-#[derive(Clone, Debug)]
 pub(crate) enum DrawCommand {
     /// Pre-tessellated mesh data (lines, arcs, circles, polygons — all use this).
-    Mesh { data: MeshData },
+    Mesh(macroquad::prelude::Mesh),
     /// Text rendering command.
     Text {
         text: String,

turtle-lib/src/tessellation.rs

@@ -3,7 +3,6 @@
 //! This module provides helper functions to tessellate paths using Lyon,
 //! which replaces the manual triangulation with GPU-optimized tessellation.
 
-use crate::state::MeshData;
 use lyon::math::{point, Point};
 use lyon::path::{LineCap, LineJoin, Path};
 use lyon::tessellation::{
@@ -31,13 +30,13 @@ pub(crate) struct SimpleVertex {
     pub(crate) position: [f32; 2],
 }
 
-/// Build mesh data from Lyon tessellation
+/// Build mesh from Lyon tessellation
 #[must_use]
-pub(crate) fn build_mesh_data(
+pub(crate) fn build_mesh(
     vertices: &[SimpleVertex],
     indices: &[u16],
     color: Color,
-) -> MeshData {
+) -> Mesh {
     let verts: Vec<Vertex> = vertices
         .iter()
         .map(|v| Vertex {
@@ -53,9 +52,10 @@ pub(crate) fn build_mesh_data(
         })
         .collect();
 
-    MeshData {
+    Mesh {
         vertices: verts,
         indices: indices.to_vec(),
+        texture: None,
     }
 }
 
@@ -69,7 +69,7 @@ pub(crate) fn build_mesh_data(
 pub(crate) fn tessellate_polygon(
     vertices: &[Vec2],
     color: Color,
-) -> Result<MeshData, Box<dyn std::error::Error>> {
+) -> Result<Mesh, Box<dyn std::error::Error>> {
     if vertices.is_empty() {
         return Err("No vertices provided".into());
     }
@@ -96,7 +96,7 @@ pub(crate) fn tessellate_polygon(
         }),
     )?;
 
-    Ok(build_mesh_data(
+    Ok(build_mesh(
         &geometry.vertices,
         &geometry.indices,
         color,
@@ -114,7 +114,7 @@ pub(crate) fn tessellate_polygon(
 pub(crate) fn tessellate_multi_contour(
     contours: &[Vec<Vec2>],
     color: Color,
-) -> Result<MeshData, Box<dyn std::error::Error>> {
+) -> Result<Mesh, Box<dyn std::error::Error>> {
     if contours.is_empty() {
         return Err("No contours provided".into());
     }
@@ -195,7 +195,7 @@ pub(crate) fn tessellate_multi_contour(
         }
     }
 
-    Ok(build_mesh_data(
+    Ok(build_mesh(
         &geometry.vertices,
         &geometry.indices,
         color,
@@ -212,7 +212,7 @@ pub(crate) fn tessellate_stroke(
     color: Color,
     width: f32,
     closed: bool,
-) -> Result<MeshData, Box<dyn std::error::Error>> {
+) -> Result<Mesh, Box<dyn std::error::Error>> {
     if vertices.is_empty() {
         return Err("No vertices provided".into());
     }
@@ -241,7 +241,7 @@ pub(crate) fn tessellate_stroke(
         }),
     )?;
 
-    Ok(build_mesh_data(
+    Ok(build_mesh(
         &geometry.vertices,
         &geometry.indices,
         color,
@@ -259,7 +259,7 @@ pub(crate) fn tessellate_circle(
     color: Color,
     filled: bool,
     stroke_width: f32,
-) -> Result<MeshData, Box<dyn std::error::Error>> {
+) -> Result<Mesh, Box<dyn std::error::Error>> {
     let mut builder = Path::builder();
     builder.add_circle(to_lyon_point(center), radius, lyon::path::Winding::Positive);
     let path = builder.build();
@@ -286,7 +286,7 @@ pub(crate) fn tessellate_circle(
         )?;
     }
 
-    Ok(build_mesh_data(
+    Ok(build_mesh(
         &geometry.vertices,
         &geometry.indices,
         color,
@@ -308,7 +308,7 @@ pub(crate) fn tessellate_arc(
     stroke_width: f32,
     segments: usize,
     direction: crate::circle_geometry::CircleDirection,
-) -> Result<MeshData, Box<dyn std::error::Error>> {
+) -> Result<Mesh, Box<dyn std::error::Error>> {
     use crate::circle_geometry::arc_points;
 
     let start_angle = start_angle_degrees.to_radians();
@@ -352,7 +352,7 @@ pub(crate) fn tessellate_arc(
         }),
     )?;
 
-    Ok(build_mesh_data(
+    Ok(build_mesh(
         &geometry.vertices,
         &geometry.indices,
         color,