perf: replace Rust WebSocket relay with direct Bun WS server for ExtHost IPC

[j4rviscmd]

Summary

Remove the Rust WebSocket relay layer and have Bun start its own WebSocket server that the WebView connects to directly. This eliminates an unnecessary hop (WebView → WS → Rust relay → pipe → Bun becomes WebView → WS → Bun), reducing RPC round-trip latency for all extension host communication.

Closes #505

Related Issue

Closes #505 — parent epic: #504

Changes

• DELETE: ws_relay.rs, handshake.rs, protocol.rs, init_data.rs — entire Rust relay layer removed
• NEW: exthostWsServer.ts — BunWsSocket ISocket implementation + startWsServer() using Bun.serve() WebSocket
• MODIFY: sidecar.rs — remove IPC pipe creation, pipe stdout for port reading via EXTHOST_WS_PORT:<port> protocol
• MODIFY: spawn_exthost.rs — simplify to direct spawn + port read, remove relay logic
• MODIFY: mod.rs — remove deleted modules, simplify ExtHostError enum
• MODIFY: extensionHostProcess.ts — add WS server mode branch with isWsServerMode() check
• MODIFY: tauriLocalProcessExtensionHost.ts — use spawn_exthost command directly, connect WebSocket to Bun's port
• MODIFY: lib.rs — update command registration (remove spawn_exthost_with_relay)
• MODIFY: Cargo.toml — remove tokio-tungstenite and futures-util dependencies

How to Test

1. Build and launch the app: npm run tauri:devwin
2. Verify the Extension Host activates successfully (check logs for [TauriExtHost] Received Initialized — handshake complete!)
3. Verify extensions load and function correctly (try opening a file, using TypeScript IntelliSense, running a terminal)
4. Verify clean shutdown — Bun process should terminate when the app exits
5. Check that the Rust logs show WS port=<port> and no relay-related errors