Fix sentinel error aliasing, hot-path allocations, and resource leaks

- Deduplicate sentinel errors: httpx.ErrNoHealthy, ErrCircuitOpen, and
  ErrRetryExhausted are now aliases to the canonical sub-package values
  so errors.Is works across package boundaries
- Retry transport returns ErrRetryExhausted only when all attempts are
  actually exhausted, not on early policy exit
- Balancer: pre-parse endpoint URLs at construction, replace req.Clone
  with cheap shallow struct copy to avoid per-request allocations
- Circuit breaker: Load before LoadOrStore to avoid allocating a Breaker
  on every request for known hosts
- Health checker: drain response body before close for connection reuse,
  probe endpoints concurrently, run initial probe synchronously in Start
- Client: add Close() to shut down health checker goroutine, propagate
  URL resolution errors instead of silently discarding them
- MockClock: fix lock ordering in Reset (clock.mu before t.mu), fix
  timer slice compaction to avoid backing-array aliasing, extract
  fireExpired to deduplicate Advance/Set

circuitbreaker/breaker.go

@@ -156,7 +156,10 @@ func Transport(opts ...Option) middleware.Middleware {
 		return middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
 			host := req.URL.Host
 
-			val, _ := hosts.LoadOrStore(host, NewBreaker(opts...))
+			val, ok := hosts.Load(host)
+			if !ok {
+				val, _ = hosts.LoadOrStore(host, NewBreaker(opts...))
+			}
 			cb := val.(*Breaker)
 
 			done, err := cb.Allow()