Usability & Ergonomics Improvements

All issues identified across Sections 3.1, 3.2, and 3.3 have been
resolved, verified with unit tests, automated compilation checks under
`-D float_literal_f32_fallback`, and headless SVG export runs.

-
  **[angle.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/general/angle.rs)**:
  - Implemented `From<f64>` and `From<usize>` for `Degrees`.
  - Added unit tests `from_integer` and `from_f64` to verify conversion
    accuracy.
-
  **[fontsize.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/general/fontsize.rs)**:
  - Implemented `From<f64>` for `FontSize`.
  - Refactored `FontSize::value(self)` to pass Copy type by value.
  - Added unit test `font_size_conversions`.
-
  **[general.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/general.rs)**:
  - Implemented `From<f64>`, `From<i32>`, and `From<usize>` for
    `AnimationSpeed`.
  - Added unit test `animation_speed_conversions`.
  - Re-exported `macroquad` crate (`pub use macroquad;`) so downstream
    code and macro expansions have reliable direct access.

-
  **[export.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/export.rs)**:
  - Made `parse_svg_export_arg()` public.
  - Implemented `run_headless_svg_export<F>(mut build_commands: F,
    filename: &str) -> Result<(), ExportError>` that executes commands
    using `app.step_animations()`, avoiding all window/GUI dependencies
    and never calling `std::process::exit`.
  - Updated `handle_svg_export` to delegate to
    `run_headless_svg_export`.
-
  **[lib.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/lib.rs)**:
  - Extracted `pub fn step_animations(&mut self)` from `update(&mut
    self)`, allowing command queue draining and tween stepping
    headlessly without querying window mouse position or events.
-
  **[state.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/state.rs)**:
  - Changed `TurtleWorld::new()` to initialize camera with
    `Camera2D::default()` instead of querying `screen_width()` /
    `screen_height()`, eliminating panics when running without a
    Macroquad window.
-
  **[turtle-lib-macros/src/lib.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib-macros/src/lib.rs)**:
  - Added `validate_input` helper providing clean compile diagnostics
    with spans for:
    - Multiple arguments: `#[turtle_main] functions must take either 0
      arguments or a single &mut TurtlePlan`
    - Non-unit return types: `#[turtle_main] functions cannot have a
      return type`
    - Async functions: `#[turtle_main] functions cannot be async`
  - Replaced `#[macroquad::main]` wrapper expansion with a native `fn
    main()` that inspects CLI arguments first. If `--export-svg` is
    present, it runs `run_headless_svg_export` directly and returns
    cleanly without opening a window. Otherwise, it launches
    `macroquad::Window::new(#window_title, async { ... })`.
  - Added 5 unit tests in `turtle-lib-macros` testing signature
    validation.

-
  **[.vscode/launch.json](file:///home/dietrich/Projekte/Source/turtlers/.vscode/launch.json)**:
  - Removed stale references to nonexistent `turtle-example` and
    `turtle-ui`.
  - Added debug configurations for `turtle-lib` tests,
    `turtle-lib-macros` tests, `hello_turtle`, and `breadboard`.
-
  **[breadboard.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/examples/breadboard.rs)**:
  - Refactored example to use `#[turtle_main("Breadboard")]`.
  - Removed `#[cfg(feature = "svg")]` and the early-exit message; the
    example now renders directly on screen by default and supports
    `--features svg -- --export-svg breadboard.svg`.
-
  **[README.md](file:///home/dietrich/Projekte/Source/turtlers/README.md)**:
  - Documented optional user-level `~/.cargo/config.toml` mold/lld
    fast-linking configuration under "Building and Running".

---

```bash
cargo test --workspace
```
- **Result**: 21 passed (16 in `turtle-lib`, 5 in `turtle-lib-macros`),
  34 doctests passed, 0 failed.

```bash
RUSTFLAGS="-D float_literal_f32_fallback" cargo check --workspace
--all-targets --all-features
RUSTFLAGS="-D float_literal_f32_fallback" cargo check --package
turtle-lib --examples --all-features
```
- **Result**: Passed with 0 errors and 0 fallback warnings across all
  workspace crates and all 30 examples.

```bash
cargo run --package turtle-lib --example hello_turtle --features svg --
--export-svg hello.svg
cargo run --package turtle-lib --example breadboard --features svg --
--export-svg breadboard.svg
```
- **Result**: Both exported SVG files successfully and exited with code
  0 without creating or flashing a graphical window.

```bash
cargo check --package turtle-lib --example breadboard
```
- **Result**: Compiled cleanly with 0 errors when SVG feature is
  disabled.

turtle-lib/examples/breadboard.rs

@@ -1,48 +1,11 @@
+//! Breadboard circuit diagram example.
+//!
+//! Demonstrates structured procedural drawing of an electronics solderless breadboard.
+//! Run normally to display on screen, or export to SVG with:
+//! `cargo run --package turtle-lib --example breadboard --features svg -- --export-svg breadboard.svg`
+
 use turtle_lib::*;
 
-#[cfg(feature = "svg")]
-#[macroquad::main("Export SVG")]
-async fn main() {
-    // Create turtle plan
-    let mut turtle = create_turtle_plan();
-
-    // Set instant mode so commands execute immediately
-    turtle.set_speed(1200).set_pen_width(0.5);
-
-    breadboard(&mut turtle, 65);
-
-    turtle.hide();
-    let mut app = TurtleApp::new().with_commands(turtle.build());
-    use macroquad::{
-        input::{is_key_pressed, KeyCode},
-        text::draw_text,
-        window::{clear_background, next_frame},
-    };
-
-    loop {
-        clear_background(WHITE);
-        app.update();
-        app.render();
-
-        draw_text("Press E for SVG export", 20.0, 40.0, 32.0, BLACK);
-
-        if is_key_pressed(KeyCode::E) {
-            match app.export_drawing("test.svg", export::DrawingFormat::Svg) {
-                Ok(_) => println!("SVG exported to test.svg"),
-                Err(e) => eprintln!("Export error: {:?}", e),
-            }
-        }
-
-        next_frame().await;
-    }
-}
-
-#[cfg(not(feature = "svg"))]
-fn main() {
-    println!("SVG export is not enabled. Build with --features svg");
-}
-
-#[cfg(feature = "svg")]
 fn pin(t: &mut TurtlePlan, size: f32) {
     t.left(90.0).forward(size / 2.0);
     for _ in 0..5 {
@@ -51,7 +14,6 @@ fn pin(t: &mut TurtlePlan, size: f32) {
     t.right(90.0).forward(size / 2.0).left(90.0);
 }
 
-#[cfg(feature = "svg")]
 fn pin_row(t: &mut TurtlePlan, count: usize) {
     for x in 0..count {
         pin(t, 5.0);
@@ -61,7 +23,6 @@ fn pin_row(t: &mut TurtlePlan, count: usize) {
     }
 }
 
-#[cfg(feature = "svg")]
 fn pin_column(t: &mut TurtlePlan, count: usize, x_coord: f32) {
     for x in 0..count {
         t.pen_up().go_to(vec2(x_coord, x as f32 * 10.0)).pen_down();
@@ -69,7 +30,6 @@ fn pin_column(t: &mut TurtlePlan, count: usize, x_coord: f32) {
     }
 }
 
-#[cfg(feature = "svg")]
 fn pin_side(t: &mut TurtlePlan, count: usize, x_coord: f32, color: Color) {
     t.pen_up()
         .go_to(vec2(x_coord, -2.5))
@@ -84,8 +44,7 @@ fn pin_side(t: &mut TurtlePlan, count: usize, x_coord: f32, color: Color) {
     }
 }
 
-#[cfg(feature = "svg")]
-fn breadboard(t: &mut TurtlePlan, row_count: usize) {
+fn draw_breadboard(t: &mut TurtlePlan, row_count: usize) {
     pin_column(t, row_count, 0.0);
     pin_column(t, row_count, 65.0);
     pin_side(t, row_count, -15.0, BLUE);
@@ -105,3 +64,10 @@ fn breadboard(t: &mut TurtlePlan, row_count: usize) {
         .forward(170.0)
         .right(90.0);
 }
+
+#[turtle_main("Breadboard")]
+fn main(turtle: &mut TurtlePlan) {
+    turtle.set_speed(1200).set_pen_width(0.5);
+    draw_breadboard(turtle, 65);
+    turtle.hide();
+}