feat(02-03): redesign WebSocket protocol — browser tick-driven beat detection

- Remove server-side broadcast_loop (MPV position polling eliminated)
- Browser sends {type: tick, position: N} at ~10Hz
- Server looks up nearest beat within 50ms window, sends {type: beat, position, tempo}
- Browser sends play/pause/seek events (logged, no UDP yet)
- Browser sends {type: load, path} to switch beat cache to specific file
- _find_nearest_beat() uses bisect for O(log n) lookup
- Per-connection beat state: current_file, beats list, tempo, last_beat_reported

lightsync/api/ws.py

@@ -1,9 +1,33 @@
-import asyncio
+"""WebSocket hub — browser-driven tick/beat protocol (Phase 02-03 redesign).
+
+Browser sends position ticks at ~10Hz; server looks up the next beat and
+sends a beat notification back when one is near.
+
+Protocol (browser → server):
+    {"type": "tick",  "position": 42.3}          — audio playback position
+    {"type": "play"}                               — playback started (log only for now)
+    {"type": "pause"}                              — playback paused (log only for now)
+    {"type": "seek",  "position": 42.3}           — seek event (log only for now)
+
+Protocol (server → browser):
+    {"type": "beat",  "position": 42.3, "tempo": 120.0}  — beat detected near position
+    {"type": "error", "message": "..."}                   — error response
+"""
+from __future__ import annotations
+
+import bisect
 import json
+import logging
 from fastapi import APIRouter, WebSocket, WebSocketDisconnect
 
+logger = logging.getLogger(__name__)
+
 router = APIRouter()
 
+# Beat detection tolerance window: consider a beat "hit" if audio position
+# is within this many seconds of a known beat timestamp.
+BEAT_WINDOW_SEC = 0.05  # 50ms
+
 
 class ConnectionManager:
     def __init__(self):
@@ -32,54 +56,99 @@ class ConnectionManager:
 manager = ConnectionManager()
 
 
-async def broadcast_loop(get_engine) -> None:
-    """Broadcast playback state to all WebSocket clients at 10Hz (AUD-04).
+def _find_nearest_beat(beats: list[float], position: float) -> float | None:
+    """Return the nearest beat timestamp to position, or None if beats is empty.
 
-    Args:
-        get_engine: callable returning the MPVEngine instance (avoids import cycle)
+    Uses bisect for O(log n) lookup.
     """
-    while True:
-        await asyncio.sleep(0.1)
-        engine = get_engine()
-        if engine and manager.active_connections:
-            state = engine.get_state()
-            await manager.broadcast({"type": "tick", **state})
+    if not beats:
+        return None
+    idx = bisect.bisect_left(beats, position)
+    candidates = []
+    if idx < len(beats):
+        candidates.append(beats[idx])
+    if idx > 0:
+        candidates.append(beats[idx - 1])
+    return min(candidates, key=lambda b: abs(b - position))
 
 
 @router.websocket("/ws")
 async def websocket_endpoint(websocket: WebSocket):
     await manager.connect(websocket)
+    # Per-connection state: which file's beats are we tracking
+    current_file: str | None = None
+    beats: list[float] = []
+    tempo: float = 0.0
+    last_beat_reported: float | None = None  # avoid re-reporting same beat
+
     try:
         while True:
             data = await websocket.receive_text()
-            msg = json.loads(data)
-            msg_type = msg.get("type")
-
-            # Import engine reference from main (module-level var)
-            from lightsync.main import engine
-
-            if engine is None:
-                await websocket.send_text(json.dumps({"type": "error", "message": "engine not ready"}))
+            try:
+                msg = json.loads(data)
+            except json.JSONDecodeError:
+                await websocket.send_text(json.dumps({"type": "error", "message": "invalid JSON"}))
                 continue
 
-            if msg_type == "play":
-                engine.play()
-                await websocket.send_text(json.dumps({"type": "ack", "command": "play"}))
+            msg_type = msg.get("type")
+
+            if msg_type == "tick":
+                position = float(msg.get("position", 0.0))
+
+                # Lazily load beat data from app state when available
+                if not beats:
+                    beats_cache = getattr(websocket.app.state, "beats", None) or {}
+                    # Use the most recently cached file if we don't have one yet
+                    if beats_cache:
+                        # Pick the last uploaded file (most likely what's playing)
+                        latest_path = list(beats_cache.keys())[-1]
+                        beat_data = beats_cache[latest_path]
+                        current_file = latest_path
+                        beats = sorted(beat_data.get("beats", []))
+                        tempo = float(beat_data.get("tempo", 0.0))
+
+                if beats:
+                    nearest = _find_nearest_beat(beats, position)
+                    if nearest is not None and abs(nearest - position) <= BEAT_WINDOW_SEC:
+                        # Only send each beat once per playback pass
+                        if last_beat_reported != nearest:
+                            last_beat_reported = nearest
+                            await websocket.send_text(json.dumps({
+                                "type": "beat",
+                                "position": nearest,
+                                "tempo": tempo,
+                            }))
+
+            elif msg_type == "play":
+                logger.info("[ws] play event at position=%.3f", float(msg.get("position", 0)))
+                last_beat_reported = None  # reset beat tracking on play
+
             elif msg_type == "pause":
-                engine.pause()
-                await websocket.send_text(json.dumps({"type": "ack", "command": "pause"}))
+                logger.info("[ws] pause event at position=%.3f", float(msg.get("position", 0)))
+
             elif msg_type == "seek":
-                position = float(msg.get("position", 0))
-                engine.seek(position)
-                await websocket.send_text(json.dumps({"type": "ack", "command": "seek", "position": position}))
+                position = float(msg.get("position", 0.0))
+                logger.info("[ws] seek to position=%.3f", position)
+                last_beat_reported = None  # reset so beats near seek target fire
+
             elif msg_type == "load":
+                # Browser signals which file is now playing — update beat cache lookup
                 path = msg.get("path", "")
                 if path:
-                    engine.load(path)
-                    await websocket.send_text(json.dumps({"type": "ack", "command": "load", "path": path}))
-                else:
-                    await websocket.send_text(json.dumps({"type": "error", "message": "missing path"}))
+                    beats_cache = getattr(websocket.app.state, "beats", None) or {}
+                    if path in beats_cache:
+                        current_file = path
+                        beat_data = beats_cache[path]
+                        beats = sorted(beat_data.get("beats", []))
+                        tempo = float(beat_data.get("tempo", 0.0))
+                        last_beat_reported = None
+                        logger.info("[ws] loaded beat data for %s: %d beats @ %.1f BPM", path, len(beats), tempo)
+                    else:
+                        logger.warning("[ws] no beat data cached for %s", path)
+
             else:
+                # Unknown message type — echo back for debugging
                 await websocket.send_text(json.dumps({"type": "ack", "echo": msg}))
+
     except WebSocketDisconnect:
         manager.disconnect(websocket)