feat(net): CircuitBreaker resilience layer (Slice 1, PR 4)

CHANGELOG.md

@@ -71,6 +71,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   exhaustion; body-transparent (drops a superseded response, releasing its `Guarded`
   permit). Adds the `Retryable` marker + `RetryConfig` schedule; jitter via an internal
   seeded `SplitMix64` — no new dependency. (ADR-0031 §2, ADR-0034.)
+- `oath-adapter-net-http-api` `CircuitBreaker` resilience layer (Slice 1 PR 4) — the
+  `CircuitBreaker<S, T>` service + `CircuitBreakerLayer<T>` factory (`net-api::Layer`):
+  the reactive backstop to `RateLimit`. Trips Open after `failure_threshold` consecutive
+  `Connection`/`Timeout`/`5xx` failures, or immediately on a `Throttled`/429 with the long
+  `throttle_cooldown`; fast-rejects with a new non-retryable `HttpError::CircuitOpen`
+  (mapped to a new `ErrorKind::CircuitOpen`) without touching the inner stack; admits
+  bounded Half-Open probes after cooldown (reached-host closes, failure re-opens). Pure
+  clock-injected `Breaker` state machine (Closed/Open/Half-Open) behind a thin
+  `Arc<Mutex<Breaker>>` + `Timer` shell; single per-host breaker; `now()`-only (no sleep,
+  no new dependency). 4-class outcome partition so `4xx`/`Auth`/unclassified errors neither
+  trip nor mask an outage. (ADR-0031 §5, ADR-0034.)
 - net-http construction-surface design refinements (ADR-0034 append-only
   Amendments 2026-07-04, spec updated) — an absent `RateLimit<K>` directive now
   **fails closed** (not "defaults to `Global`"), closing the last silent

crates/adapter/net/api/src/error_kind.rs

@@ -33,6 +33,12 @@ pub enum ErrorKind {
 
     /// The error does not fit any other category.
     Unknown,
+
+    /// A circuit breaker rejected the request without sending it — the breaker is
+    /// Open after prior failures (or a throttle) and is failing fast until its
+    /// cooldown elapses. A deliberate local decision, not a transport outcome;
+    /// non-retryable.
+    CircuitOpen,
 }
 
 /// Implemented by error types that can be classified as an [`ErrorKind`].