refactor: phase 4 - event-driven architecture foundation

- Add EventBus class with pub/sub messaging (thread-safe)
- Add emitter Protocol classes (EventEmitter, Startable, Stoppable)
- Add event emission to NtfyPoller (NtfyMessageEvent)
- Add event emission to MicMonitor (MicLevelEvent)
- Update StreamController to publish stream start/end events
- Add comprehensive tests for eventbus and emitters modules

engine/scroll.py

@@ -43,11 +43,15 @@ def stream(items, ntfy_poller, mic_monitor):
     scroll_motion_accum = 0.0
     msg_cache = (None, None)
 
-    while queued < config.HEADLINE_LIMIT or active:
+    while True:
+        if queued >= config.HEADLINE_LIMIT and not active:
+            break
+
         t0 = time.monotonic()
         w, h = tw(), th()
         fh = config.FIREHOSE_H if config.FIREHOSE else 0
         ticker_view_h = h - fh
+        scroll_step_interval = calculate_scroll_step(config.SCROLL_DUR, ticker_view_h)
 
         msg = ntfy_poller.get_active_message()
         msg_overlay, msg_cache = render_message_overlay(msg, w, h, msg_cache)