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-lock.json

@@ -16,11 +16,11 @@
         "react-dom": "^18.2.0",
         "react-router-dom": "^6.20.0",
         "react-simple-code-editor": "^0.14.1",
-        "sonner": "^1.7.4",
         "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",
@@ -5469,16 +5469,6 @@
         "node": ">=8"
       }
     },
-    "node_modules/sonner": {
-      "version": "1.7.4",
-      "resolved": "https://registry.npmjs.org/sonner/-/sonner-1.7.4.tgz",
-      "integrity": "sha512-DIS8z4PfJRbIyfVFDVnK9rO3eYDtse4Omcm6bt0oEr5/jtLgysmjuBl1frJ9E/EQZrFmKx2A8m/s5s9CRXIzhw==",
-      "license": "MIT",
-      "peerDependencies": {
-        "react": "^18.0.0 || ^19.0.0 || ^19.0.0-rc",
-        "react-dom": "^18.0.0 || ^19.0.0 || ^19.0.0-rc"
-      }
-    },
     "node_modules/source-map-js": {
       "version": "1.2.1",
       "resolved": "https://registry.npmjs.org/source-map-js/-/source-map-js-1.2.1.tgz",
@@ -6324,6 +6314,16 @@
         "xterm": "^5.0.0"
       }
     },
+    "node_modules/xterm-addon-webgl": {
+      "version": "0.16.0",
+      "resolved": "https://registry.npmjs.org/xterm-addon-webgl/-/xterm-addon-webgl-0.16.0.tgz",
+      "integrity": "sha512-E8cq1AiqNOv0M/FghPT+zPAEnvIQRDbAbkb04rRYSxUym69elPWVJ4sv22FCLBqM/3LcrmBLl/pELnBebVFKgA==",
+      "deprecated": "This package is now deprecated. Move to @xterm/addon-webgl instead.",
+      "license": "MIT",
+      "peerDependencies": {
+        "xterm": "^5.0.0"
+      }
+    },
     "node_modules/yallist": {
       "version": "3.1.1",
       "resolved": "https://registry.npmjs.org/yallist/-/yallist-3.1.1.tgz",