feat(espnow): broadcast delivery with group-filtered routing

Send presets and select on broadcast with groups; unicast only for
per-device settings. V1 select as [preset_id, step?]. Sequence steps
use beat counts; manual presets get select each beat, auto only on
step change. Bridge downlink router, Pi envelope delivery, and tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/test_sequence_step_beats.py

@@ -0,0 +1,51 @@
+"""Sequence step ``beats`` hold (e.g. 12 beats on preset 42, then 4 on preset 5)."""
+
+import asyncio
+import os
+import sys
+
+PROJECT_ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+SRC_PATH = os.path.join(PROJECT_ROOT, "src")
+if SRC_PATH not in sys.path:
+    sys.path.insert(0, SRC_PATH)
+
+from util import sequence_playback as sp  # noqa: E402
+
+
+def test_step_holds_beats_before_lane_send(monkeypatch):
+    sent = []
+
+    async def fake_send_lane(i, st, ctx):
+        sent.append((int(st.get("stepIdx", 0)), int(st.get("beatCount", 0))))
+
+    monkeypatch.setattr(sp, "_send_lane", fake_send_lane)
+
+    async def noop_stop(**_kwargs):
+        with sp._beat_run_lock:
+            sp._beat_run = None
+
+    monkeypatch.setattr(sp, "stop_playback", noop_stop)
+
+    ctx = {
+        "num_lanes": 1,
+        "loop": False,
+        "lanes": [[{"preset_id": "42", "beats": 12}, {"preset_id": "5", "beats": 4}]],
+        "lane_states": [{"stepIdx": 0, "beatCount": 0, "done": False}],
+        "sequence_loop_beat": 0,
+    }
+    with sp._beat_run_lock:
+        sp._beat_run = ctx
+
+    async def run():
+        for _ in range(11):
+            await sp.process_active_beat_advance()
+        await sp.process_active_beat_advance()
+        for _ in range(3):
+            await sp.process_active_beat_advance()
+        await sp.process_active_beat_advance()
+
+    asyncio.run(run())
+    assert sent == [(1, 0)]
+    assert ctx["lane_states"][0]["done"] is True
+    with sp._beat_run_lock:
+        sp._beat_run = None