fix: per-workstream SSE fan-out — multiple consumers no longer steal … (#38)

* fix: per-workstream SSE fan-out — multiple consumers no longer steal each other's tokens

After 4d665a5 removed the single-consumer SSE lock, the shared
_event_queue let concurrent consumers (browser, bridge, console proxy)
race on Queue.get(), each receiving ~1/N of content tokens and producing
garbled streaming text.

Replace the single queue with per-client fan-out: each SSE connection
registers its own bounded queue (maxsize=500) on WebUI._listeners, and
_enqueue() copies every event to all registered queues. On eviction or
close, a ws_closed sentinel is injected so SSE generators exit promptly.

* fix: address CI failures and Copilot review feedback

- Handle ws_closed sentinel in events_sse generator (break on close)
- Guarantee sentinel delivery by evicting one item when queue is full
- Clear listeners list after injecting sentinels on cleanup
- Fix test_slow_consumer to fill only slow queue directly
- Fix ruff SIM117 (nested with), unused import, mypy unused-ignore

docs/api-reference.md

@@ -460,13 +460,13 @@ The server sends an SSE comment every 5 seconds when no events are pending:
 This prevents proxies and browsers from closing the connection due to
 inactivity.
 
-#### Generation mechanism
+#### Multi-consumer fan-out
 
-Each new SSE connection to a workstream increments an internal
-`_sse_generation` counter. The previous SSE handler detects the generation
-mismatch and exits its event loop, ensuring only one active SSE connection per
-workstream at a time. The event queue is drained of stale events before the new
-connection begins streaming.
+Each SSE connection to a workstream receives its own delivery queue.  Events
+produced by the worker thread are fanned out to all registered listener queues,
+so multiple consumers (browser, bridge, console proxy, SDK) can connect
+simultaneously and each receives every event.  On reconnect the client receives
+a full history replay, so no catch-up mechanism is needed.
 
 ---