Add 4CM routing fields to Preset data model

- Add routing_mode (mono/4cm) and routing_breakpoint (default 7) to Preset
- Serialize/deserialize in _preset_to_dict / _preset_from_dict
- Backward compatible — old presets default to mono/7
- Add get_4cm_config() / set_4cm_config() helpers on PresetManager
- set_4cm_config validates mode and saves back to disk
- All 48 related tests pass (46 preset + 2 integration activation)

src/presets/manager.py

@@ -44,6 +44,8 @@ def _preset_to_dict(preset: Preset) -> dict:
         "bank": preset.bank,
         "program": preset.program,
         "master_volume": preset.master_volume,
+        "routing_mode": preset.routing_mode,
+        "routing_breakpoint": preset.routing_breakpoint,
         "tuner_enabled": preset.tuner_enabled,
         "chain": [
             {
@@ -99,6 +101,8 @@ def _preset_from_dict(data: dict) -> Preset:
         chain=chain,
         midi_mappings=midi_mappings,
         master_volume=data.get("master_volume", 0.8),
+        routing_mode=data.get("routing_mode", "mono"),
+        routing_breakpoint=data.get("routing_breakpoint", 7),
         tuner_enabled=data.get("tuner_enabled", False),
     )
 
@@ -332,6 +336,38 @@ class PresetManager:
 
         logger.info("Reordered preset at (%d,%d) → (%d,%d)", bank, program, bank, new_program)
 
+    # ── 4CM routing config ──────────────────────────────────────────────
+
+    def get_4cm_config(self, preset: Preset) -> tuple[str, int]:
+        """Return (routing_mode, routing_breakpoint) from a preset.
+
+        Returns:
+            Tuple of ``(routing_mode, routing_breakpoint)``.
+        """
+        return (preset.routing_mode, preset.routing_breakpoint)
+
+    def set_4cm_config(
+        self, preset: Preset, mode: str, breakpoint: int = 7
+    ) -> None:
+        """Set 4CM routing configuration on a preset in-place and save it.
+
+        Args:
+            preset: The preset to modify.
+            mode: ``\"mono\"`` or ``\"4cm\"``.
+            breakpoint: Chain index where the pre/post split occurs
+                (default 7 = after IR_CAB, before mod FX).
+
+        Raises:
+            ValueError: If ``mode`` is not ``\"mono\"`` or ``\"4cm\"``.
+        """
+        if mode not in ("mono", "4cm"):
+            raise ValueError(f"routing_mode must be 'mono' or '4cm', got {mode!r}")
+        preset.routing_mode = mode
+        preset.routing_breakpoint = breakpoint
+        self.save(preset)
+        logger.info("Set 4CM config on '%s': mode=%s breakpoint=%d",
+                     preset.name, mode, breakpoint)
+
     # ── Bank management ─────────────────────────────────────────────────────
 
     def list_banks(self) -> list[Bank]:

src/presets/types.py

@@ -60,6 +60,10 @@ class Preset:
     master_volume: float = 0.8
     tuner_enabled: bool = False
 
+    # 4-Cable Method routing
+    routing_mode: str = "mono"      # "mono" or "4cm"
+    routing_breakpoint: int = 7     # chain index where pre/post split occurs
+
 
 @dataclass
 class Bank: