Bound WatchedField change queue to prevent memory growth

[zeevdr]

Description

Both WatchedField._change_queue (sdk/src/opendecree/watcher.py:59) and AsyncWatchedField._change_queue (sdk/src/opendecree/async_watcher.py:59) use unbounded queue.Queue() / asyncio.Queue() with no maxsize. If a consumer of field.changes() falls behind while the server sends rapid updates, memory grows indefinitely.

The TypeScript SDK already handles this with DEFAULT_QUEUE_SIZE = 1024 and a drop-oldest strategy with _droppedChanges tracking (decree-typescript/src/watcher.ts:31,203-206). The Python SDK should adopt the same pattern.

Acceptance criteria

• WatchedField and AsyncWatchedField accept a max_queue_size parameter (default 1024)
• When the queue is full, the oldest entry is dropped (drop-oldest strategy)
• A dropped_changes counter is exposed on the field object
• Tests verify bounded behavior under rapid updates with a slow consumer
• Behavior matches the TypeScript SDK's queue overflow semantics