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/templates/dashboard.html

@@ -85,5 +85,30 @@
             </div>
         </div>
     </div>
+
+    <!-- 4CM Routing Status -->
+    <div class="card">
+        <div class="card-header">
+            <h2>Routing</h2>
+        </div>
+        <div class="card-body">
+            <div class="routing-status">
+                <span class="routing-indicator" id="routing-mode-indicator" data-mode="{{ routing_mode }}">
+                    {% if routing_mode == '4cm' %}
+                        <span class="badge badge-4cm">4CM</span>
+                    {% else %}
+                        <span class="badge badge-mono">Mono</span>
+                    {% endif %}
+                </span>
+                <span class="routing-info" id="routing-info">
+                    {% if routing_mode == '4cm' %}
+                        Breakpoint: {{ routing_breakpoint }}
+                    {% else %}
+                        Full chain
+                    {% endif %}
+                </span>
+            </div>
+        </div>
+    </div>
 </div>
 {% endblock %}