🧹 [code health improvement] Refactor register_graphics to use extracted functions

[Tcode-Motion]

🎯 What: Extracted the inline closure callbacks inside the massive register_graphics method in stdlib/src/graphics.rs into standalone, module-private Rust functions (create_canvas, draw_rect, draw_circle, draw_line, save_png, save_jpeg). A shared helper function, save_image_with_format, was also created to DRY up the save_png and save_jpeg implementations.
💡 Why: register_graphics was excessively long and difficult to read due to housing multiple lengthy function implementations within Rc::new(StdFunction { ... }) blocks. This refactoring greatly improves the maintainability and readability of register_graphics and reduces code duplication in the image saving logic.
✅ Verification: Verified the exact same logical behavior is preserved. Ran cargo test -p techscript_stdlib test_graphics_canvas_drawing and cargo test --workspace to ensure all tests passed and no regressions were introduced. Also ran cargo fmt --all.
✨ Result: A much cleaner, more concise register_graphics function, reduced code duplication for image formatting, and overall improved code health in the graphics.rs file.

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PR created automatically by Jules for task 9581920255701488411 started by @Tcode-Motion

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