Add collaboration support

Real-time collaboration through consumer-provided transport and
presence interfaces. Also includes a sample backend app.

examples/flask-collab/README.md

@@ -0,0 +1,46 @@
+# Flask Collaboration Example
+
+A minimal Flask server demonstrating ribbit's collaboration features:
+real-time sync, presence, locking, and revisions.
+
+## Setup
+
+```sh
+pip install flask flask-sock
+```
+
+Copy (or symlink) the ribbit dist into the static directory:
+
+```sh
+ln -s /path/to/ribbit/dist/ribbit static/ribbit
+```
+
+## Run
+
+```sh
+python server.py
+```
+
+Open http://localhost:5000 in multiple browser tabs. Edits in one tab
+appear in the others in real time.
+
+## What it demonstrates
+
+- **Real-time sync**: WebSocket relays document updates between clients
+- **Presence**: colored badges show connected users and their status
+- **Revisions**: save button creates named revisions, click to restore
+- **Locking**: (available via console: `editor.lockForEditing()`)
+- **Source mode**: entering markdown mode pauses sync, shows remote change count
+
+## Architecture
+
+```
+Browser A ──┐
+            ├── WebSocket ──→ Flask server ──→ WebSocket ──→ Browser B
+Browser C ──┘                     │
+                                  ├── /api/revisions (REST)
+                                  └── /api/lock (REST)
+```
+
+The server is ~160 lines. In production you'd replace the in-memory
+stores with a database and add authentication.