🧹 [Extract Canvas API implementations]

[Tcode-Motion]

🎯 What: Extracted the verbose inline instantiations of CanvasFn structs inside register_canvas into a dedicated build_canvas_api helper function.
💡 Why: Reduces duplicate code, significantly shortens register_canvas, and abstracts the interior mutability (buffer.clone()) setup logic for canvas capabilities, improving readability and maintainability.
✅ Verification: Visually verified with sed that the extraction successfully maintained all exports. Compiled safely under cargo check -p techscript_stdlib without introducing new failures (pre-existing workspace failures are unchanged).
✨ Result: Cleaned up ~160 lines of repetitive setup into a readable 35-line structure.

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PR created automatically by Jules for task 9712651533992335162 started by @Tcode-Motion

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