feat(03-01): create MetricsCollector and wire into gateway

- Add MetricsCollector class with counters, model call ring buffer, event ring buffer, and active request tracking
- Add system.metrics, system.events, system.activeRequests RPC handlers
- Add GET /health unauthenticated HTTP endpoint for Docker HEALTHCHECK
- Add totalPending() to LaneQueue for queue depth metrics
- Add 20 tests for MetricsCollector

src/gateway/lane-queue.ts

@@ -67,6 +67,15 @@ export class LaneQueue {
     return this.lanes.get(laneId)?.queue.length ?? 0;
   }
 
+  /** Get the total number of pending items across all lanes. */
+  totalPending(): number {
+    let total = 0;
+    for (const lane of this.lanes.values()) {
+      total += lane.queue.length;
+    }
+    return total;
+  }
+
   /**
    * Cancel all pending entries in a lane.
    * Active work is NOT interrupted — only queued items are rejected.