Add adapter implementation guide

docs/ADAPTERS.md

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+# Writing Editor Adapters for collabd
+
+This guide explains how to write an adapter that lets any editor participate
+in collaborative editing sessions.
+
+## Architecture
+
+```
+┌─────────────┐     json/channel     ┌─────────────┐     websocket     ┌─────────────┐
+│   Editor    │ ◄──────────────────► │   Bridge    │ ◄────────────────► │   Daemon    │
+│  (plugin)   │                      │   (bun)     │                    │   :4040     │
+└─────────────┘                      └─────────────┘                    └─────────────┘
+```
+
+The **bridge** is a Bun process that:
+- Manages the Yjs document (CRDT state)
+- Translates editor buffer changes to Yjs operations
+- Translates remote Yjs updates to buffer content
+- Speaks a simple JSON protocol with the editor plugin
+
+The **plugin** is editor-specific code that:
+- Hooks buffer change events
+- Sends buffer content to bridge
+- Applies remote content from bridge
+- Optionally displays peer cursors
+
+## Why a Bridge?
+
+Most editors can't embed Yjs directly:
+- Vim: No npm/node, limited async
+- Helix: Rust-only plugins
+- Kakoune: Shell-based scripting
+
+The bridge isolates CRDT complexity. Plugins stay simple.
+
+## Protocol: Plugin ↔ Bridge
+
+Messages are newline-delimited JSON.
+
+### Plugin → Bridge
+
+```json
+{"type": "connect", "room": "myroom"}
+{"type": "disconnect"}
+{"type": "content", "text": "full buffer contents"}
+{"type": "cursor", "line": 5, "col": 10}
+```
+
+### Bridge → Plugin
+
+```json
+{"type": "ready"}
+{"type": "connected", "room": "myroom"}
+{"type": "disconnected"}
+{"type": "content", "text": "full buffer contents"}
+{"type": "peers", "count": 2}
+{"type": "cursor", "data": {"clientId": 123, "line": 5, "col": 10, "name": "peer-123"}}
+{"type": "error", "message": "connection failed"}
+```
+
+## Implementing a Plugin
+
+### 1. Spawn the Bridge
+
+```
+bun run adapters/vim/bridge.ts
+```
+
+Or use your own bridge (see next section).
+
+Communication via stdin/stdout with JSON lines.
+
+### 2. Wait for Ready
+
+Bridge sends `{"type": "ready"}` when it starts. Then send connect:
+
+```json
+{"type": "connect", "room": "myroom"}
+```
+
+### 3. Handle Content Updates
+
+When you receive `{"type": "content", "text": "..."}`:
+
+1. Save cursor position
+2. Replace buffer contents with received text
+3. Restore cursor position (clamped to valid range)
+
+**Critical:** Do NOT trigger your change handler when applying remote content,
+or you'll create an infinite loop.
+
+### 4. Send Local Changes
+
+When user edits the buffer:
+
+1. Get full buffer text
+2. Send `{"type": "content", "text": "full text"}`
+
+The bridge handles diffing - just send the whole buffer.
+
+### 5. Cursor Sync (Optional)
+
+On cursor move:
+```json
+{"type": "cursor", "line": 5, "col": 10}
+```
+
+On receiving cursor:
+```json
+{"type": "cursor", "data": {"line": 5, "col": 10, "name": "peer"}}
+```
+
+Render a highlight/virtual text at that position.
+
+## Implementing a Custom Bridge
+
+If you need a bridge in a different language (e.g., Rust for Helix),
+implement the same protocol. Key responsibilities:
+
+1. **Connect to daemon** at `ws://localhost:4040/ws`
+
+2. **Manage Y.Doc** - create on connect, apply updates
+
+3. **Diff buffer changes** - when receiving content from plugin:
+   ```
+   old_text = current Y.Text content
+   new_text = received buffer content
+   diff = compute_diff(old, new)
+   apply diff as Y.Text operations
+   ```
+
+4. **Forward updates** - when Y.Doc changes from remote:
+   ```
+   send full text to plugin as {"type": "content", ...}
+   ```
+
+5. **Handle awareness** - forward cursor positions both ways
+
+### Yjs Libraries by Language
+
+- **JavaScript/TypeScript:** `yjs` (npm)
+- **Rust:** `yrs` (crates.io) - Yjs port
+- **Python:** `pycrdt` (pypi)
+- **Go:** `yjs-go` (experimental)
+
+## Reference: Vim Adapter
+
+See `adapters/vim/` for a complete example:
+
+- `collab.vim` - Vim9script plugin
+- `bridge.ts` - TypeScript bridge process
+
+The vim plugin:
+1. Spawns bridge as job
+2. Communicates via channels (vim's async IPC)
+3. Sets TextChanged autocmd to detect edits
+4. Applies remote content with `:delete` + `setline()`
+
+## Testing Your Adapter
+
+1. Start daemon: `just dev`
+2. Open editor A, join room "test"
+3. Open editor B (vim works), join room "test"
+4. Type in one - should appear in other within 100ms
+5. Type in both simultaneously - should converge
+
+## Common Pitfalls
+
+**Feedback loops:** Applying remote content triggers your change handler,
+which sends it back. Use a flag to suppress during remote apply.
+
+**Cursor jitter:** Cursor moves during remote apply. Save/restore position.
+
+**Large files:** Sending full buffer on every keystroke is fine for <1MB.
+For larger files, consider debouncing or incremental updates.
+
+**Encoding:** Always UTF-8. Line endings should be `\n`.