feat(audio): Focusrite 2i2 stereo JACK config for 4CM

Adds:
- AudioConfig.mode field: 'mono' (default) or 'stereo_4cm'
- capture_channels / playback_channels properties for dynamic JACK -i/-o counts
- FOCUSRITE_PROFILES dict with focusrite_2i2_3gen entry
- AudioSystem.channel_mapping_help() static method
- stereo_4cm port auto-connect wiring (4 cable method)
- Systemd service content uses dynamic channel counts
- Default config YAML includes 'mode: mono'
- 11 new tests (31 total pass)
- Focusrite 4CM wiring docs in docs/config-audio.md

docs/config-audio.md

@@ -291,4 +291,68 @@ raspi-gpio get 18 19 20 21
 | 5V     | —        | Pins 2, 4    | Power        | RPi → HAT |
 | GND    | —        | Pins 6, 9, 14, 25, 30, 34, 39 | Ground | — |
 
-The I2S HAT uses a **stacking header** (2×20 female socket) to pass through all 40 GPIO pins so footswitches, LEDs, and display are accessible from the top of the HAT.
+The I2S HAT uses a **stacking header** (2×20 female socket) to pass through all 40 GPIO pins so footswitches, LEDs, and display are accessible from the top of the HAT.
+
+---
+
+## Focusrite Scarlett 2i2 — 4CM (Four Cable Method)
+
+> **Interface:** Focusrite Scarlett 2i2 (3rd gen) USB audio interface
+> **Mode:** `stereo_4cm` in `audio.mode`
+> **Device:** `hw:1,0` (USB card index, may vary)
+
+### Wiring
+
+```
+Guitar ─────→ Input 1 (ADC channel 0) ─→ hw:1,0 capture_0 ─→ Pipeline input_0
+Amp FX Send ─→ Input 2 (ADC channel 1) ─→ hw:1,0 capture_1 ─→ Pipeline input_1
+
+Pipeline output_0 (pre-amp) ─→ Output 1 (DAC channel 0) ─→ hw:1,0 playback_0 ─→ Amp Input (Send)
+Pipeline output_1 (post-amp) ─→ Output 2 (DAC channel 1) ─→ hw:1,0 playback_1 ─→ Amp FX Return
+```
+
+### JACK Config
+
+```bash
+# Kill PulseAudio first — it grabs USB audio devices
+pulseaudio --kill
+
+# Start JACK at 48kHz/128 frames, 2 capture + 2 playback channels
+jackd -R -d alsa -d hw:1,0 -r 48000 -p 128 -n 3 -C 2 -P 2
+```
+
+### Application Config
+
+Set these fields in `~/.pedal/config.yaml`:
+
+```yaml
+audio:
+  hat_type: "focusrite_2i2_3gen"
+  profile: "standard"
+  mode: "stereo_4cm"
+  input_device: "hw:1,0"
+  output_device: "hw:1,0"
+  jack_enabled: true
+  auto_connect: true
+```
+
+### Port Auto-Connect
+
+When `auto_connect: true` and mode is `stereo_4cm`, the system creates these JACK connections:
+
+| Source | Destination | Signal |
+|--------|-------------|--------|
+| `system:capture_1` | `fx_in:input_0` | Guitar → pre-amp DSP |
+| `system:capture_2` | `fx_in:input_1` | FX Send → post-amp DSP |
+| `fx_out:output_0` | `system:playback_1` | Pre-amp DSP → amp input |
+| `fx_out:output_1` | `system:playback_2` | Post-amp DSP → FX return |
+
+### Channel Mapping Helper
+
+At runtime, call `AudioSystem.channel_mapping_help("stereo_4cm")` to get these mappings as structured data.
+
+### Notes
+
+- The Focusrite 2i2 registers as ALSA card index **1** when the I2S HAT is card 0 (Linux arranges by driver probe order). If HDMI audio or other USB devices are present, the index may shift — use `aplay -l` / `arecord -l` to confirm.
+- 2-in/2-out at 24-bit/48kHz. JACK runs at 24-bit by default over ALSA.
+- At 48kHz/128 frames × 3 periods, round-trip latency is ~6-8ms — well within the 10ms target for real-time guitar DSP.