Add multi-track playback with mute/solo support

Replaces the single-root-pattern sequencer with a Track[] array that
allows multiple patterns to loop independently. Adds ADD_TRACK (0x0A),
REMOVE_TRACK (0x0B), PLAY_TRACKS (0x0C), and SET_TRACK_MUTE (0x0D)
protocol records. The C backend gains per-track pending_subs and a
tracks_mutex. The Node server gains track-state APIs (/api/tracks/:id/
active, mute, solo) and the frontend shows per-pattern track/mute/solo
buttons in the sidebar list.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

protocol.yaml

@@ -119,6 +119,38 @@ records:
         type: uint16
         note: Note-off will be sent after this many milliseconds
 
+  ADD_TRACK:
+    id: 0x0A
+    direction: node_to_c
+    description: Add a pattern as an independently-looping track
+    fields:
+      - name: pattern_id
+        type: uint16
+
+  REMOVE_TRACK:
+    id: 0x0B
+    direction: node_to_c
+    description: Remove a track and stop its playback
+    fields:
+      - name: pattern_id
+        type: uint16
+
+  PLAY_TRACKS:
+    id: 0x0C
+    direction: node_to_c
+    description: Start the multi-track engine with all added tracks
+    fields: []
+
+  SET_TRACK_MUTE:
+    id: 0x0D
+    direction: node_to_c
+    description: Mute or unmute a track without resetting its position
+    fields:
+      - name: pattern_id
+        type: uint16
+      - name: muted
+        type: uint8
+
   ACK:
     id: 0x81
     direction: c_to_node