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/src/components/terminal.tsx

@@ -8,6 +8,7 @@ import React, {
 import { Terminal } from "xterm";
 import { FitAddon } from "xterm-addon-fit";
 import { WebLinksAddon } from "xterm-addon-web-links";
+import { WebglAddon } from "xterm-addon-webgl";
 import "xterm/css/xterm.css";
 
 import {
@@ -107,6 +108,7 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 
 			// WebSocket connection established
 			const ws = new WebSocket(wsUrl);
+			ws.binaryType = "arraybuffer";
 			wsRef.current = ws;
 
 			ws.onopen = () => {
@@ -137,14 +139,35 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 				}, 30000);
 			};
 
+			// Flow control: accumulate processed bytes and send ack
+			let ackAccumulator = 0;
+			const ACK_THRESHOLD = 4096;
+			let ackTimeout: ReturnType<typeof setTimeout> | null = null;
+
+			const flushAck = () => {
+				if (ackAccumulator > 0 && ws.readyState === WebSocket.OPEN) {
+					ws.send(JSON.stringify({ type: "ack", chars: ackAccumulator }));
+					ackAccumulator = 0;
+				}
+			};
+
 			ws.onmessage = (event) => {
 				if (!termRef.current) return;
 
-				if (event.data instanceof Blob) {
-					event.data.arrayBuffer().then((buffer) => {
-						const data = new Uint8Array(buffer);
-						termRef.current?.write(data);
-					});
+				if (event.data instanceof ArrayBuffer) {
+					const data = new Uint8Array(event.data);
+					termRef.current.write(data);
+
+					// Flow control: accumulate processed bytes
+					ackAccumulator += data.length;
+					if (ackAccumulator >= ACK_THRESHOLD) {
+						flushAck();
+					} else if (!ackTimeout) {
+						ackTimeout = setTimeout(() => {
+							ackTimeout = null;
+							flushAck();
+						}, 100);
+					}
 				} else if (typeof event.data === "string") {
 					try {
 						const msg = JSON.parse(event.data);
@@ -251,6 +274,11 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 				lineHeight: 1.2,
 				letterSpacing: 0,
 				allowTransparency: false,
+				scrollback: 10000,
+				ignoreBracketedPasteMode: false,
+				fastScrollSensitivity: 5,
+				scrollSensitivity: 1,
+				smoothScrollDuration: 0,
 				theme: {
 					background: "#1e1e1e",
 					foreground: "#d4d4d4",
@@ -282,6 +310,20 @@ export const TerminalComponent = React.forwardRef<TerminalRef, TerminalProps>(
 			term.loadAddon(fitAddon);
 			term.loadAddon(new WebLinksAddon());
 
+			// Load WebGL renderer for GPU acceleration, fall back to DOM
+			try {
+				const webglAddon = new WebglAddon();
+				term.loadAddon(webglAddon);
+				webglAddon.onContextLoss(() => {
+					console.warn("WebGL context lost, falling back to DOM renderer");
+					webglAddon.dispose();
+					// Trigger a refit since cell dimensions may differ
+					requestAnimationFrame(() => fitTerminal());
+				});
+			} catch (e) {
+				console.warn("WebGL renderer failed to load, using DOM renderer", e);
+			}
+
 			const container = terminalRef.current;
 
 			// Define fitTerminal before connectWebSocket so it's available in onmessage