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

[Spirotot]

Summary

• Adds a WebSocket endpoint (/ws/sessions/:id/terminal) as a low-latency bidirectional channel for terminal I/O, replacing per-keystroke HTTP POST + SSE output with a single persistent connection
• Strictly additive — the existing HTTP POST input and SSE terminal output paths remain fully functional; the frontend opts into WS when available and falls back transparently
• Server-side 8ms micro-batching with 16KB flush threshold groups rapid PTY events into single WS frames
• Each WS batch is wrapped in DEC 2026 synchronized update markers (\x1b[?2026h / \x1b[?2026l) so xterm.js renders atomically — eliminates flicker from Ink's cursor-up redraws (Ink's DA capability negotiation for DEC 2026 fails through the PTY→server→WS proxy chain, so server-injected markers are required)

Protocol

All JSON text frames:

Direction	Type	Payload
Server → Client	{"t":"o","d":"..."}	Terminal output (DEC 2026 wrapped)
Server → Client	{"t":"c"}	Clear terminal
Server → Client	{"t":"r"}	Needs refresh (reload buffer)
Client → Server	{"t":"i","d":"..."}	Input (keystroke or paste)
Client → Server	{"t":"z","c":N,"r":N}	Resize terminal

Files changed

• src/web/routes/ws-routes.ts (new) — WebSocket route handler with micro-batching and DEC 2026 wrapping
• src/web/routes/index.ts — Barrel export
• src/web/server.ts — @fastify/websocket registration
• src/web/public/app.js — WS client lifecycle, SSE suppression guards, input/resize fast paths
• package.json — @fastify/websocket dependency

Test plan

• Verify WS connects on session select (Safari, Chrome)
• Verify input latency is dramatically lower vs HTTP POST
• Verify SSE fallback works when WS is unavailable
• Verify DEC 2026 markers present in WS output (console diagnostic)
• Verify no flicker during Ink cursor-up redraws
• Verify WS disconnects on tab switch and session delete
• Verify resize messages sent via WS fast path

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