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-macros/src/lib.rs

@@ -259,7 +259,7 @@ fn turtle_main_impl(
                         return;
                     }
                     Err(e) => {
-                        eprintln!("Error exporting SVG: {:?}", e);
+                        eprintln!("Error exporting SVG: {}", e);
                         std::process::exit(1);
                     }
                 }