Wire 4CM routing: JackAudioClient + boot orchestration + set_routing_mode()

- Add JackAudioClient class to src/system/audio.py — real-time JACK
  client owning ports and a process callback that streams audio through
  the DSP pipeline. Handles mono (1ch I/O) and stereo 4CM (2ch I/O).
- Wire PedalApp.boot(): read audio.mode from config, set pipeline
  routing_mode (mono/4cm), create and start JackAudioClient with
  matching channel count.
- Add PedalApp.set_routing_mode(mode) — hot-switch between mono and
  stereo_4cm at runtime: updates pipeline.routing_mode, restarts JACK
  client with new channel count via set_channel_count(), re-connects
  JACK ports.
- Wire shutdown: stop JackAudioClient before MIDI/footswitch.
- Export JackAudioClient from src.system.__init__.
- 53 tests pass (22 integration + 31 audio).

main.py

@@ -22,7 +22,7 @@ from typing import Optional
 import yaml
 
 # ── Subsystem imports ─────────────────────────────────────────────────────────
-from src.system.audio import AudioConfig, AudioSystem, _jack_is_running
+from src.system.audio import AudioConfig, AudioSystem, JackAudioClient, _jack_is_running
 from src.dsp.pipeline import AudioPipeline
 from src.dsp.nam_host import NAMHost
 from src.dsp.ir_loader import IRLoader
@@ -77,6 +77,7 @@ class PedalApp:
         # ── Subsystem references (set during boot) ─────────────────────
         self.audio_config: AudioConfig | None = None
         self.audio_system: AudioSystem | None = None
+        self.jack_audio: JackAudioClient | None = None
         self.nam_host: NAMHost | None = None
         self.ir_loader: IRLoader | None = None
         self.pipeline: AudioPipeline | None = None
@@ -110,9 +111,11 @@ class PedalApp:
         try:
             # ── 1. Audio config + system ──────────────────────────
             acfg = self._config["audio"]
+            audio_mode = acfg.get("mode", "mono")
             self.audio_config = AudioConfig(
                 hat_type=acfg.get("hat_type", "audioinjector"),
                 profile=acfg.get("profile", "standard"),
+                mode=audio_mode,
                 input_device=acfg.get("input_device", "hw:0,0"),
                 output_device=acfg.get("output_device", "hw:0,0"),
                 jack_enabled=acfg.get("jack_enabled", True),
@@ -130,6 +133,32 @@ class PedalApp:
             self.ir_loader = IRLoader()
             self.pipeline = AudioPipeline(nam_host=self.nam_host, ir_loader=self.ir_loader)
 
+            # Set pipeline routing from config mode
+            if audio_mode == "stereo_4cm":
+                self.pipeline.routing_mode = "4cm"
+                logger.info("Pipeline routing set to 4CM mode")
+            else:
+                self.pipeline.routing_mode = "mono"
+
+            # ── 2b. JACK audio client (capture → pipeline → playback) ──
+            if self.audio_config.jack_enabled:
+                is_stereo = audio_mode == "stereo_4cm"
+                channels = 2 if is_stereo else 1
+                self.jack_audio = JackAudioClient(
+                    pipeline=self.pipeline,
+                    client_name="pi-multifx",
+                    input_channels=channels,
+                    output_channels=channels,
+                )
+                self.jack_audio.start()
+                if not self.jack_audio._active:
+                    logger.warning("JACK audio client failed to start — running without real-time audio I/O")
+                else:
+                    logger.info(
+                        "JACK audio client started: %s mode, %d ch I/O",
+                        audio_mode, channels,
+                    )
+
             # ── 3. Preset manager ─────────────────────────────────
             pcfg = self._config["presets"]
             self.presets = PresetManager(
@@ -390,6 +419,43 @@ class PedalApp:
             self.leds.set_pixel(0, (0, 64, 255))  # Blue = active
         logger.info("Active: '%s' (bank=%d, pg=%d)", preset.name, preset.bank, preset.program)
 
+    # ═══════════════════════════════════════════════════════════════
+    # Audio routing mode switching
+    # ═══════════════════════════════════════════════════════════════
+
+    def set_routing_mode(self, mode: str) -> None:
+        """Hot-switch between mono and stereo_4cm routing at runtime.
+
+        Updates the pipeline routing mode, then restarts the JACK
+        audio client with the appropriate channel count.
+
+        Args:
+            mode: ``"mono"`` or ``"stereo_4cm"``.
+
+        Raises:
+            ValueError: If *mode* is not recognised.
+        """
+        if mode not in ("mono", "stereo_4cm"):
+            raise ValueError(f"set_routing_mode: unknown mode {mode!r}")
+
+        if self.audio_config is not None:
+            self.audio_config.mode = mode
+
+        if self.pipeline is not None:
+            self.pipeline.routing_mode = "4cm" if mode == "stereo_4cm" else "mono"
+            logger.info("Pipeline routing mode switched to %s", self.pipeline.routing_mode)
+
+        # Restart JACK client with new channel count
+        if self.jack_audio is not None:
+            channels = 2 if mode == "stereo_4cm" else 1
+            self.jack_audio.set_channel_count(channels, channels)
+
+        # Re-connect JACK ports for the new routing
+        if self.audio_system is not None and self.audio_config is not None:
+            self.audio_system.connect_fx_ports()
+
+        logger.info("Routing mode switched to %s", mode)
+
     # ═══════════════════════════════════════════════════════════════
     # Display sync
     # ═══════════════════════════════════════════════════════════════
@@ -500,7 +566,15 @@ class PedalApp:
         logger.info("═══════ Shutting down ═══════")
         shutdown_start = time.monotonic()
 
-        # 1. Stop accepting input (MIDI + footswitch)
+        # 1. Stop JACK audio client (before JACK server)
+        if self.jack_audio:
+            try:
+                self.jack_audio.stop()
+                logger.debug("JACK audio client stopped")
+            except Exception as e:
+                logger.warning("JACK audio client stop error: %s", e)
+
+        # 2. Stop accepting input (MIDI + footswitch)
         if self.midi:
             try:
                 self.midi.stop()