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

tests/test_audio_system.py

@@ -10,6 +10,7 @@ from system.audio import (
     AudioSystem,
     LATENCY_PROFILES,
     I2S_CONFIGS,
+    FOCUSRITE_PROFILES,
     CONFIG_TXT,
     JACK_SERVICE_PATH,
     LIMITS_CONF,
@@ -187,6 +188,73 @@ def test_systemd_service_content_custom_device():
     assert "-dhw:2,0" in content
 
 
+# ── Mode / stereo_4cm tests ───────────────────────────────────────
+
+
+def test_audio_config_mode_default():
+    cfg = AudioConfig()
+    assert cfg.mode == "mono"
+
+
+def test_audio_config_mode_stereo_4cm():
+    cfg = AudioConfig(mode="stereo_4cm")
+    assert cfg.mode == "stereo_4cm"
+
+
+def test_audio_config_channel_counts_mono():
+    cfg = AudioConfig(mode="mono")
+    assert cfg.capture_channels == 1
+    assert cfg.playback_channels == 1
+
+
+def test_audio_config_channel_counts_stereo_4cm():
+    cfg = AudioConfig(mode="stereo_4cm")
+    assert cfg.capture_channels == 2
+    assert cfg.playback_channels == 2
+
+
+def test_audio_config_focusrite_profile_hit():
+    cfg = AudioConfig(hat_type="focusrite_2i2_3gen")
+    prof = cfg.focusrite_profile
+    assert prof is not None
+    desc, cap, play = prof
+    assert "Focusrite" in desc
+    assert cap == 2
+    assert play == 2
+
+
+def test_audio_config_focusrite_profile_miss():
+    cfg = AudioConfig()
+    assert cfg.focusrite_profile is None
+
+
+def test_focusrite_profiles_entries():
+    for key, (desc, cap, play) in FOCUSRITE_PROFILES.items():
+        assert len(desc) > 10
+        assert isinstance(cap, int) and cap > 0
+        assert isinstance(play, int) and play > 0
+
+
+def test_channel_mapping_help_mono():
+    mapping = AudioSystem.channel_mapping_help("mono")
+    assert len(mapping) == 1
+    assert mapping[0]["capture_port"] == "system:capture_1"
+    assert mapping[0]["playback_port"] == "system:playback_1"
+
+
+def test_channel_mapping_help_stereo_4cm():
+    mapping = AudioSystem.channel_mapping_help("stereo_4cm")
+    assert len(mapping) == 2
+    assert "Guitar" in mapping[0]["description"]
+    assert "FX Send" in mapping[1]["description"]
+
+
+def test_channel_mapping_help_unknown():
+    import pytest
+    with pytest.raises(ValueError, match="Unknown mode"):
+        AudioSystem.channel_mapping_help("bogus")
+
+
 # ── Module exports ────────────────────────────────────────────────