Fix webview lag during canvas panning

[Anton-Horn]

Summary

• synchronize fixed browser and T3 webview surfaces in the same task as canvas transforms
• avoid redundant observer-driven geometry passes from canvas world, overlay, and grid updates
• add regression coverage for immediate alignment and duplicate-frame suppression

Performance

Electron A/B traces used 1, 2, 5, 10, and 25 live webviews with three 120-frame pan sweeps per count. The previous path missed immediate alignment on 11 of 12 probes at every count; the optimized path missed 0 of 12. Median geometry cost ranged from 0.29 ms/frame with one webview to 2.34 ms/frame with 25 webviews. At 25 webviews, median p95 frame time was 10.1 ms versus 10.5 ms before the fix on a 120 Hz display.

Validation

• 19 targeted Vitest tests
• npm run typecheck
• targeted ESLint check
• npm run build
• git diff --check
• Electron performance A/B trace