4CM split routing: stereo pipeline + breakpoint + Web UI

Pipeline:
- process() dispatches to _process_mono() or _process_4cm() based on routing_mode
- _process_4cm() splits chain at routing_breakpoint: pre blocks on ch0, post on ch1
- _process_single_block() extracted for reuse in both mono and 4cm paths
- routing_mode/routing_breakpoint load from preset via load_preset()
- set_routing() for runtime configuration
- Properties: routing_mode (mono|4cm), routing_breakpoint with validation

Web server:
- GET /api/routing — current routing mode and breakpoint
- POST /api/routing — set routing mode/breakpoint, persist to current preset, WS broadcast
- _gather_state() includes routing_mode and routing_breakpoint

Web UI:
- Settings page: 4CM toggle + breakpoint slider with routing description
- Dashboard: routing badge (4CM/Mono) with breakpoint info
- app.js: WebSocket handler, updateRoutingUI(), toggle4cm(), set4cmBreakpoint()
- style.css: .badge, .badge-4cm, .badge-mono, .routing-status, .routing-desc

Tests: mock pipeline fixture updated with routing_mode/routing_breakpoint

src/web/server.py

@@ -356,6 +356,53 @@ class WebServer:
             pl._master_volume = vol
             return {"ok": True, "volume": vol}
 
+        # ── 4CM routing endpoints ────────────────────────────────
+
+        @app.get("/api/routing")
+        async def get_routing():
+            """Get current routing mode and breakpoint."""
+            pl = self.deps.pipeline
+            if not pl:
+                raise HTTPException(status_code=503)
+            return {
+                "routing_mode": pl.routing_mode,
+                "routing_breakpoint": pl.routing_breakpoint,
+            }
+
+        @app.post("/api/routing")
+        async def set_routing(data: dict):
+            """Set routing mode and/or breakpoint.
+
+            Body:
+                routing_mode (optional): ``\"mono\"`` or ``\"4cm\"``.
+                routing_breakpoint (optional): chain index for split.
+            """
+            pl = self.deps.pipeline
+            pm = self.deps.presets
+            if not pl:
+                raise HTTPException(status_code=503)
+            mode = data.get("routing_mode", pl.routing_mode)
+            bp = int(data.get("routing_breakpoint", pl.routing_breakpoint))
+            pl.set_routing(mode, bp)
+
+            # Persist to current preset if preset manager is available
+            if pm:
+                try:
+                    bank = pm.current_bank
+                    program = pm.current_program
+                    preset = pm.load(bank, program)
+                    pm.set_4cm_config(preset, mode=mode, breakpoint=bp)
+                except Exception:
+                    pass
+
+            await self._manager.broadcast({
+                "type": "routing_changed",
+                "routing_mode": pl.routing_mode,
+                "routing_breakpoint": pl.routing_breakpoint,
+            })
+            return {"ok": True, "routing_mode": pl.routing_mode,
+                    "routing_breakpoint": pl.routing_breakpoint}
+
         @app.get("/api/models")
         async def list_models():
             """List available NAM models."""
@@ -578,6 +625,8 @@ class WebServer:
             "bypass": pl._bypassed if pl else False,
             "tuner_enabled": pl._tuner_enabled if pl else False,
             "master_volume": pl._master_volume if pl else 0.8,
+            "routing_mode": pl.routing_mode if pl else "mono",
+            "routing_breakpoint": pl.routing_breakpoint if pl else 7,
             "nam_loaded": bool(nam and nam.is_loaded) if nam else False,
             "nam_model": current_model_name,
             "ir_loaded": bool(ir and ir.is_loaded) if ir else False,