Add per-host circuit breaker with three-state machine

Implements circuit breaker as a RoundTripper middleware:
- Closed → Open after consecutive failure threshold
- Open → HalfOpen after configurable duration
- HalfOpen → Closed on success, back to Open on failure
- Per-host tracking via sync.Map for independent endpoint isolation

circuitbreaker/options.go

@@ -0,0 +1,50 @@
+package circuitbreaker
+
+import "time"
+
+type options struct {
+	failureThreshold int           // consecutive failures to trip
+	openDuration     time.Duration // how long to stay open before half-open
+	halfOpenMax      int           // max concurrent requests in half-open
+}
+
+func defaults() options {
+	return options{
+		failureThreshold: 5,
+		openDuration:     30 * time.Second,
+		halfOpenMax:      1,
+	}
+}
+
+// Option configures a Breaker.
+type Option func(*options)
+
+// WithFailureThreshold sets the number of consecutive failures required to
+// trip the breaker from Closed to Open. Default is 5.
+func WithFailureThreshold(n int) Option {
+	return func(o *options) {
+		if n > 0 {
+			o.failureThreshold = n
+		}
+	}
+}
+
+// WithOpenDuration sets how long the breaker stays in the Open state before
+// transitioning to HalfOpen. Default is 30s.
+func WithOpenDuration(d time.Duration) Option {
+	return func(o *options) {
+		if d > 0 {
+			o.openDuration = d
+		}
+	}
+}
+
+// WithHalfOpenMax sets the maximum number of concurrent probe requests
+// allowed while the breaker is in the HalfOpen state. Default is 1.
+func WithHalfOpenMax(n int) Option {
+	return func(o *options) {
+		if n > 0 {
+			o.halfOpenMax = n
+		}
+	}
+}