⚡ Bolt: Optimize circuit breaker fast path to prevent DashMap write lock contention

.jules/bolt.md

@@ -56,3 +56,6 @@
 ## 2025-10-30 - [Avoid HashMap Allocation in Tree Indexing]
 **Learning:** In the `cancel_subtree` function within `orch8-engine/src/evaluator.rs`, allocating a `HashMap<ParentId, Vec<ChildId>>` to index the tree for a BFS traversal introduces massive hashing and memory allocation overhead on the cancellation hot path, resulting in O(N²) overall cost for deep trees when combining the allocation and iteration per node.
 **Action:** Replace dynamic parent-to-children index maps (`HashMap<ParentId, Vec<ChildId>>`) with a flat `Vec<(ParentId, ChildId)>` sorted by parent ID. This allows using `.partition_point()` to perform O(log N) zero-allocation lookups for children on every visited node.
+## 2026-06-24 - [DashMap Lock Contention & Key Allocation in Cascading Failures]
+**Learning:** When a circuit breaker is already in an `Open` state during a failure storm, simply incrementing the failure count with a write lock via `.get_mut(q)` causes unnecessary `DashMap` contention across the shard. The `failure_count` isn't actively evaluated when the breaker is already `Open` (only the `opened_at` timestamp matters for the `HalfOpen` transition check).
+**Action:** Always implement a fast-path read lock using `.get()` in high-throughput updates to a concurrent `DashMap` when mutations are functionally no-ops (like adding failures to an already open breaker). Return early to completely avoid the write lock.

orch8-engine/src/circuit_breaker.rs

@@ -306,6 +306,16 @@ impl CircuitBreakerRegistry {
         let now = Utc::now();
         let search = KeyRef(tenant_id, handler);
         let q: &dyn CircuitKey = &search;
+
+        // ⚡ Bolt: Fast path read lock to avoid write-lock contention. If the circuit
+        // is already fully Open, there is no need to increment the failure count
+        // or acquire a write lock on the DashMap shard.
+        if let Some(breaker) = self.breakers.get(q) {
+            if breaker.state == BreakerState::Open {
+                return;
+            }
+        }
+
         let mut tripped_snapshot = None;
 
         if let Some(mut breaker) = self.breakers.get_mut(q) {