Add README

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

README.md

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+# midi-sequencer
+
+A browser-based MIDI step sequencer with a C ALSA backend for accurate timing and a Node.js/Express web UI.
+
+## Architecture
+
+```
+Browser (app.mjs)
+    ↕  HTTP + SSE
+Node server (server.mjs, port 3000)
+    ↕  Unix socket (/tmp/midi-sequencer.sock)
+C backend (midi-sequencer binary)
+    ↕  ALSA sequencer API
+MIDI output
+```
+
+- **`protocol.yaml`** — single source of truth for the binary protocol; drives codegen for both C and Node
+- **`codegen/gen.mjs`** — generates `c-backend/generated/protocol.{h,c}` and `node-server/src/generated/protocol.mjs`
+- **`c-backend/`** — C11, drift-free tick thread using `clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME)`, links libasound + libpthread
+- **`node-server/`** — Express 5 web app, serves the UI and bridges the browser to the C backend
+
+## Features
+
+- **Multi-track playback** — up to 16 patterns playing simultaneously, each looping at its own independent length
+- **Live track control** — add/remove patterns from the playing set without stopping; mute and solo per track
+- **GM percussion editor** — channel 10 patterns show GM drum names in drum-machine layout with family colour coding
+- **Auto-save** — step grid edits are sent to the backend immediately on each toggle
+- **Preview notes** — per-row ▶ button auditions a note without affecting sequencer state
+- **Click-to-rename** — note and percussion labels are editable inline
+- **Sub-patterns** — patterns can reference other patterns as sub-sequences at any step
+- **BPM** — tempo settable to 0.1 BPM resolution (stored as `bpm_x10` uint16, no floats in protocol)
+
+## Protocol
+
+Binary frames: `[record_type: u8][payload_length: u16le][payload…]`
+
+| ID | Name | Direction | Description |
+|---|---|---|---|
+| 0x01 | HELLO | node→C | Version handshake |
+| 0x02 | DEFINE_PATTERN | node→C | Create/redefine a pattern |
+| 0x03 | CLEAR_PATTERN | node→C | Remove all notes from a pattern |
+| 0x04 | ADD_NOTE | node→C | Add a note event at a step |
+| 0x05 | ADD_SUB_PATTERN | node→C | Schedule a sub-pattern within a pattern |
+| 0x06 | PLAY | node→C | Single-pattern play (backward compat) |
+| 0x07 | STOP | node→C | Stop all playback |
+| 0x08 | SET_TEMPO | node→C | Set BPM (as bpm×10) |
+| 0x09 | PREVIEW_NOTE | node→C | Play a single note immediately |
+| 0x0A | ADD_TRACK | node→C | Add a pattern to the multi-track set |
+| 0x0B | REMOVE_TRACK | node→C | Remove a pattern from the multi-track set |
+| 0x0C | PLAY_TRACKS | node→C | Start the multi-track engine |
+| 0x0D | SET_TRACK_MUTE | node→C | Mute/unmute a track |
+| 0x81 | ACK | C→node | Acknowledge a command |
+| 0x82 | ERROR | C→node | Report an error |
+| 0x83 | BEAT_TICK | C→node | Step notification (one per track per tick) |
+| 0x84 | PATTERN_END | C→node | Pattern completed one full cycle |
+
+## Build
+
+Requires: GCC, make, libasound2-dev, Node.js ≥ 18
+
+```sh
+# Full build (codegen + npm install + C binary)
+make
+
+# C backend only
+cd c-backend && make
+
+# Run the Node server
+cd node-server && make start
+# → http://localhost:3000
+```
+
+The C backend runs separately and is started independently:
+
+```sh
+./c-backend/midi-sequencer [socket_path] [alsa_client_name]
+# defaults: /tmp/midi-sequencer.sock  midi-sequencer
+```
+
+## Usage
+
+1. Start the C backend
+2. Start the Node server (`make start` in `node-server/`)
+3. Open `http://localhost:3000`
+4. Create melodic or drum patterns with **+ Melodic** / **🥁 Drums**
+5. Click the **●** dot next to each pattern to add it to the playback set
+6. Press **▶ Play** to start — all active patterns loop independently
+
+## Timing
+
+One step = one 16th note. Step duration: `150,000,000,000 ns / bpm_x10`
+
+At 120.0 BPM: 125 ms/step. The tick thread uses an absolute deadline so tempo drift does not accumulate.