feat: high-performance web terminal with asyncio-native I/O

Complete rewrite of the terminal pipeline for VS Code Server-level
responsiveness. Key improvements:

Backend:
- Replace blocking select.select(0.1) with asyncio.add_reader() for
  event-driven PTY reading (eliminates ~110ms polling latency)
- Add output batching (2ms window) to reduce WebSocket frame overhead
- Add flow control: client acks processed bytes, server pauses PTY reads
  at 64KB threshold, resumes at 32KB
- Add 5s ack timeout fallback to prevent stuck sessions

Frontend:
- Switch WebSocket to binary mode (binaryType = 'arraybuffer')
- Eliminate Blob -> arrayBuffer async conversion overhead
- Add flow control ack messages (every 4096 bytes or 100ms)
- Add xterm-addon-webgl with graceful DOM fallback
- Add performance tuning (scrollback=10000, fastScrollSensitivity)

SDD artifacts:
- openspec/explorations/terminal-responsiveness.md
- openspec/proposals/terminal-responsiveness.md
- openspec/specs/terminal-responsiveness.md
- openspec/designs/terminal-responsiveness.md
- openspec/tasks/terminal-responsiveness.md

Quality gates: pytest (19 passed, 1 skipped), tsc --noEmit clean

apps/web/package.json

@@ -22,7 +22,8 @@
     "tailwindcss": "^3.3.0",
     "xterm": "^5.3.0",
     "xterm-addon-fit": "^0.8.0",
-    "xterm-addon-web-links": "^0.9.0"
+    "xterm-addon-web-links": "^0.9.0",
+    "xterm-addon-webgl": "^0.16.0"
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",