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/api/src/api/terminal.py

@@ -222,8 +222,7 @@ async def _handle_terminal_websocket(
         # Use mutable session reference so loops can survive reset
         session_ref = SessionRef(session, slot_session_id)
 
-        # Start I/O loops and heartbeat
-        read_task = asyncio.create_task(_read_loop(session_ref, websocket))
+        # Start write loop and heartbeat (read is now event-driven in TerminalSession)
         write_task = asyncio.create_task(
             _write_loop(session_ref, websocket, instance_id)
         )
@@ -232,7 +231,7 @@ async def _handle_terminal_websocket(
 
         # Wait for either task to complete (indicating disconnect or error)
         done, pending = await asyncio.wait(
-            [read_task, write_task, heartbeat_task],
+            [write_task, heartbeat_task],
             return_when=asyncio.FIRST_COMPLETED,
         )
 
@@ -267,28 +266,6 @@ async def _handle_terminal_websocket(
                 )
 
 
-async def _read_loop(session_ref: SessionRef, websocket) -> None:
-    """Read output from the container and send to WebSocket."""
-    try:
-        while True:
-            session = session_ref.session
-            if not session.is_alive() or session._closed:
-                await asyncio.sleep(0.1)
-                continue
-            data = await session.read_output()
-            if data:
-                try:
-                    await websocket.send_bytes(data)
-                except WebSocketDisconnect:
-                    break
-                except Exception:
-                    break
-            else:
-                await asyncio.sleep(0.01)
-    except Exception:
-        pass
-
-
 async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> None:
     """Read input from WebSocket and send to container."""
     try:
@@ -319,6 +296,10 @@ async def _write_loop(session_ref: SessionRef, websocket, instance_id: str) -> N
                                     rows,
                                 )
                                 await session.resize(cols, rows)
+                            elif msg_type == "ack":
+                                char_count = ctrl.get("chars", 0)
+                                if char_count > 0:
+                                    session.acknowledge_data(char_count)
                             elif msg_type == "reset":
                                 # Reset terminal session (scoped to current slot)
                                 logger.debug(