Fix long-session canvas compositor slowdown

[Anton-Horn]

Summary

• Explicitly release territory WebGL contexts when canvas workspaces unmount or GPU initialization falls back.
• Coalesce repeated pan and zoom notifications into one territory draw per animation frame.
• Add regression coverage for context disposal, fallback cleanup, and frame coalescing.

Diagnosis

After several hours, the affected renderer sustained roughly one CPU core while its JavaScript main thread was mostly idle. Native sampling showed Chromium's compositor thread saturated, and restarting Cate reset the slowdown.

Workspace switches remount the canvas subtree, while territory teardown previously deleted GL objects without explicitly surrendering the underlying context. Pan notifications also issued synchronous territory draws, allowing redundant GPU work to queue faster than Chromium could composite it.

Validation

• Focused Vitest regressions: 4 passed.
• TypeScript typecheck passed.
• Targeted ESLint passed.
• Production build passed.

The broader sandboxed suite completed with 3,602 passing tests. Listener and file-watcher integration groups failed because the sandbox denied binding localhost with EPERM.