Headless SVG Export Macroquad Panic Clarification

Clarified and improved error reporting when drawing routines invoke
Macroquad window or rendering functions (such as `screen_width()` or
`screen_height()`) during headless SVG export (`--export-svg`).

[export.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/src/export.rs)
- Added `ExportError::Execution(String)` variant to represent drawing
  execution failures and panics.
- Implemented `std::fmt::Display` and `std::error::Error` for
  `ExportError`.
- Created `PanicHookGuard` RAII struct ensuring any installed panic
  hooks are automatically restored after execution.
- In `run_headless_svg_export`:
  - Chains onto the existing panic hook to preserve standard panic
    backtrace and line number information.
  - Detects if the panic originated from uninitialized Macroquad context
    (`THREAD_ID.is_some()`).
  - Emits a clear, prominent diagnostic banner explaining that
    window/GUI functions are unavailable in headless mode.
  - Catches the panic via `std::panic::catch_unwind` and returns
    `Err(ExportError::Execution(...))`.
  - In `handle_svg_export`, uses `{e}` (Display) rather than `{e:?}`
    (Debug) for cleaner error reporting.

[lib.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib-macros/src/lib.rs)
- Updated the expanded `main` function generated by `#[turtle_main]` to
  format errors with `{}` (Display) instead of `{:?}` (Debug).

[sierpinski_triangle.rs](file:///home/dietrich/Projekte/Source/turtlers/turtle-lib/examples/sierpinski_triangle.rs)
- Added documentation notes to the module header and
  `sierpinski_triangle_auto` noting that the example requires an active
  graphics window and cannot be exported to SVG headlessly because it
  queries window dimensions.

---

Executed:
```bash
RUST_BACKTRACE=1 cargo run --example sierpinski_triangle --features svg
-- --export-svg sier.svg
```

Output:
```
thread 'main' (49702) panicked at
/home/dietrich/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/macroquad-0.4.16/src/lib.rs:172:13:
assertion failed: THREAD_ID.is_some()
stack backtrace:
0: __rustc::rust_begin_unwind
...
5: macroquad::window::screen_width
6: sierpinski_triangle::sierpinski_triangle_auto
at ./turtle-lib/examples/sierpinski_triangle.rs:100:20
7: sierpinski_triangle::draw_sierpinski
at ./turtle-lib/examples/sierpinski_triangle.rs:39:5
...
================================================================================
Headless SVG Export Note:
A Macroquad window/rendering function (e.g. `screen_width()`,
`screen_height()`,
or input check) was called while running in headless export mode.
Headless export does not initialize a graphics window. To resolve this:
  - Use relative turtle commands or fixed coordinates instead of window
    queries, or
  - Run the program in windowed GUI mode without the `--export-svg`
    flag.
    ================================================================================

Error exporting SVG: execution error: Drawing function called Macroquad
window/GUI functions (e.g. `screen_width()`, `screen_height()`) which
are unavailable in headless SVG export mode.
```
- Process exited cleanly with exit code 1.

Executed:
```bash
cargo run --example koch --features svg -- --export-svg /tmp/koch.svg
```
Output:
```
SVG exported successfully to: /tmp/koch.svg
```
Exit code 0.

```bash
cargo test --package turtle-lib
cargo clippy --package turtle-lib --features svg -- -Wclippy::pedantic
-Aclippy::cast_precision_loss -Aclippy::cast_sign_loss
-Aclippy::cast_possible_truncation
```
- 17 unit tests + 34 doctests passed (1 ignored doctest).
- Clippy completed with zero warnings.

turtle-lib/examples/sierpinski_triangle.rs

@@ -17,6 +17,10 @@
 //! That is relatively easy to implement, as long as you follow these steps and let recursion do
 //! the rest. Another little bonus this example provides is the ability to customize the drawing
 //! size: the triangle will stay correctly sized and positioned automatically.
+//!
+//! Note: This example queries `screen_width()` and `screen_height()` to calculate positioning
+//! relative to the window. Consequently, it requires an active graphics window and does not
+//! support headless SVG export (`--export-svg`).
 
 use macroquad::window::{screen_height, screen_width};
 use turtle_lib::*;
@@ -86,6 +90,9 @@ fn sierpinski_triangle(turtle: &mut TurtlePlan, level: u8, size: f32) {
 /// `level` is still required, it can't be computed automatically. However, given the used
 /// canvas size, it will compute the appropriate size and start point so the triangle gets
 /// centered and occupies as much drawing space as possible while staying in bounds.
+///
+/// Note: Because this function queries window dimensions, it requires an active graphics
+/// window and cannot be run in headless SVG export mode.
 fn sierpinski_triangle_auto(turtle: &mut TurtlePlan, level: u8) {
     let size = TRIANGLE_SIZE;