Eliminates per-frame WASM allocations for shadows and blurs:
- Reusable effectLayerPaint instead of new Paint() per effect
- ImageFilter cache keyed by params (drop shadow, blur, decal blur)
- Zero allocations during SkPicture recording for cached effects
- Replace deflate with Zstd compression via zstd-wasm for .fig export
(Figma expects Zstd, not zlib deflate)
- Extract IS_TAURI constant from repeated window check
- Fix Tauri fs permissions: use allow-write-file / allow-read-file
with glob scope instead of bare allow-write / allow-read
