feat: Enhance canvas texture management and selection encoding
- Introduced `SharedCompositePixels` for stable full-canvas pixel storage, enabling efficient pipeline creation and recovery.
- Added `TextureUpdate` struct for tightly packed dirty rectangle uploads, reducing unnecessary data copying.
- Refactored `upload_dirty_region` to utilize `TextureUpdate`, improving performance by eliminating full buffer scans.
- Implemented `encode_selection_texture` to generate an encoded R8 selection mask, optimizing selection rendering.
- Updated shader to sample selection texture only once, reducing GPU workload.
- Added comprehensive tests for texture updates, selection encoding, and performance diagnostics.
- Documented design decisions and validation sequences to ensure future performance stability.
phantom
2d5b7a37e8