Initial scaffold: ALSA MIDI C backend + Node ESM sequencer web app

- protocol.yaml: SSoT for the binary framing protocol (12 record types)
- codegen/gen.mjs: generates C header/source and Node ESM from protocol.yaml
- c-backend: ALSA sequencer with drift-free clock_nanosleep tick thread,
  pattern store (hierarchical sub-patterns), Unix socket server
- node-server: Express 5 web app — REST API, SSE for real-time beat events,
  step-sequencer frontend with pending-edit / explicit-save flow

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

protocol.yaml

@@ -0,0 +1,147 @@
+version: 1
+description: Binary protocol between Node sequencer and C ALSA MIDI backend
+
+# Frame format: [record_type: uint8][payload_length: uint16le][payload: bytes...]
+# Header size: 3 bytes
+# High bit set (0x80+) means C → Node direction
+
+frame:
+  - name: record_type
+    type: uint8
+  - name: payload_length
+    type: uint16
+
+# Type mapping used by codegen
+types:
+  uint8:  { c_type: uint8_t,  c_put: put_u8,  c_get: get_u8,  node_write: writeUInt8,   node_read: readUInt8,   size: 1 }
+  uint16: { c_type: uint16_t, c_put: put_u16, c_get: get_u16, node_write: writeUInt16LE, node_read: readUInt16LE, size: 2 }
+
+records:
+  HELLO:
+    id: 0x01
+    direction: node_to_c
+    description: Protocol version handshake
+    fields:
+      - name: version
+        type: uint8
+
+  DEFINE_PATTERN:
+    id: 0x02
+    direction: node_to_c
+    description: Define or redefine a pattern (clears existing data)
+    fields:
+      - name: pattern_id
+        type: uint16
+      - name: steps
+        type: uint8
+        note: Total step count e.g. 16 for a one-bar pattern at 16th-note resolution
+      - name: channel
+        type: uint8
+        note: MIDI channel 0-15
+
+  CLEAR_PATTERN:
+    id: 0x03
+    direction: node_to_c
+    description: Remove all notes and sub-pattern references from a pattern
+    fields:
+      - name: pattern_id
+        type: uint16
+
+  ADD_NOTE:
+    id: 0x04
+    direction: node_to_c
+    description: Add a note event to a pattern at a specific step
+    fields:
+      - name: pattern_id
+        type: uint16
+      - name: step
+        type: uint8
+        note: 0-based step index within the pattern
+      - name: note
+        type: uint8
+        note: MIDI note number 0-127 (middle C = 60)
+      - name: velocity
+        type: uint8
+        note: 0-127
+      - name: duration_steps
+        type: uint8
+        note: Duration in steps (1 = one step)
+
+  ADD_SUB_PATTERN:
+    id: 0x05
+    direction: node_to_c
+    description: Schedule a sub-pattern to start at a given step within a parent pattern
+    fields:
+      - name: pattern_id
+        type: uint16
+        note: Parent pattern ID
+      - name: step
+        type: uint8
+        note: Step within parent at which the sub-pattern begins playing
+      - name: sub_pattern_id
+        type: uint16
+
+  PLAY:
+    id: 0x06
+    direction: node_to_c
+    description: Start playing a pattern from step 0
+    fields:
+      - name: pattern_id
+        type: uint16
+
+  STOP:
+    id: 0x07
+    direction: node_to_c
+    description: Stop all playback and send MIDI all-notes-off
+    fields: []
+
+  SET_TEMPO:
+    id: 0x08
+    direction: node_to_c
+    description: Set global tempo (applies immediately, even mid-sequence)
+    fields:
+      - name: bpm_x10
+        type: uint16
+        note: "BPM × 10 for 0.1 BPM resolution — e.g. 1200 = 120.0 BPM"
+
+  ACK:
+    id: 0x81
+    direction: c_to_node
+    description: Acknowledge a received command
+    fields:
+      - name: acked_type
+        type: uint8
+        note: Record type being acknowledged
+
+  ERROR:
+    id: 0x82
+    direction: c_to_node
+    description: Report an error condition
+    fields:
+      - name: code
+        type: uint8
+      - name: context_type
+        type: uint8
+        note: Record type that triggered this error
+
+  BEAT_TICK:
+    id: 0x83
+    direction: c_to_node
+    description: Timing notification sent on every sequencer step
+    fields:
+      - name: pattern_id
+        type: uint16
+      - name: step
+        type: uint8
+        note: Current step within the pattern (0-based)
+      - name: beat
+        type: uint8
+        note: Current beat number within the current cycle (wraps at 255)
+
+  PATTERN_END:
+    id: 0x84
+    direction: c_to_node
+    description: Notification that a pattern has completed one full cycle
+    fields:
+      - name: pattern_id
+        type: uint16