Adds a pub/sub subscription layer driven by SQLite WAL change detection, enabling reactive consumers to receive events in real time without polling the log.
v0.3.02026-06-12
·WAL watcher polls for new frames and fans out to registered listeners via crossbeam-channel
·Subscription API: subscribe(stream_id, branch) returns an async-compatible event receiver
·std::thread + crossbeam-channel threading model — no Tokio runtime required
·Subscription lifecycle: cancel, drop-on-disconnect, and idle-timeout handling