feat: add WebSocket terminal I/O with server-side DEC 2026 sync

Replace per-keystroke HTTP POST + SSE terminal output with a single
bidirectional WebSocket connection for dramatically lower input latency.
The existing SSE+POST paths remain fully functional as fallback.

Server-side: ws-routes.ts provides /ws/sessions/:id/terminal with 8ms
micro-batching and 16KB flush threshold. Each batch is wrapped in
DEC 2026 synchronized update markers so xterm.js renders atomically —
Ink's DA capability negotiation fails through the PTY→server→WS proxy
chain, so without server-injected markers, cursor-up redraws flicker.

Frontend: _connectWs/_disconnectWs manage per-session WS lifecycle.
Input and resize use WS fast path with HTTP POST fallback. SSE terminal
events are suppressed when WS is active to prevent double rendering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/server.ts

@@ -31,6 +31,7 @@ import Fastify, { FastifyInstance, FastifyReply } from 'fastify';
 import fastifyCompress from '@fastify/compress';
 import fastifyCookie from '@fastify/cookie';
 import fastifyStatic from '@fastify/static';
+import fastifyWebsocket from '@fastify/websocket';
 import { join, dirname } from 'node:path';
 import { fileURLToPath } from 'node:url';
 import { existsSync, mkdirSync, readFileSync, chmodSync } from 'node:fs';
@@ -103,6 +104,7 @@ import {
   registerRespawnRoutes,
   registerRalphRoutes,
   registerPlanRoutes,
+  registerWsRoutes,
 } from './routes/index.js';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
@@ -563,6 +565,9 @@ export class WebServer extends EventEmitter {
       this.qrAuthFailures = authState.qrAuthFailures;
     }
 
+    // WebSocket support (terminal I/O — low-latency bidirectional channel)
+    await this.app.register(fastifyWebsocket);
+
     // Security headers + CORS
     registerSecurityHeaders(this.app, this.https);
     // Service worker must never be cached — browsers check for SW updates on navigation
@@ -700,6 +705,7 @@ export class WebServer extends EventEmitter {
     registerRespawnRoutes(this.app, ctx);
     registerRalphRoutes(this.app, ctx);
     registerPlanRoutes(this.app, ctx);
+    registerWsRoutes(this.app, ctx);
   }
 
   /**