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

retry/retry.go

@@ -1,6 +1,8 @@
 package retry
 
 import (
+	"errors"
+	"fmt"
 	"io"
 	"net/http"
 	"time"
@@ -8,6 +10,10 @@ import (
 	"git.codelab.vc/pkg/httpx/middleware"
 )
 
+// ErrRetryExhausted is returned when all retry attempts have been exhausted
+// and the last attempt also failed.
+var ErrRetryExhausted = errors.New("httpx: all retry attempts exhausted")
+
 // Policy decides whether a failed request should be retried.
 type Policy interface {
 	// ShouldRetry reports whether the request should be retried. The extra
@@ -28,6 +34,7 @@ func Transport(opts ...Option) middleware.Middleware {
 		return middleware.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
 			var resp *http.Response
 			var err error
+			var exhausted bool
 
 			for attempt := range cfg.maxAttempts {
 				// For retries (attempt > 0), restore the request body.
@@ -48,6 +55,7 @@ func Transport(opts ...Option) middleware.Middleware {
 
 				// Last attempt — return whatever we got.
 				if attempt == cfg.maxAttempts-1 {
+					exhausted = true
 					break
 				}
 
@@ -85,6 +93,10 @@ func Transport(opts ...Option) middleware.Middleware {
 				}
 			}
 
+			// Wrap with ErrRetryExhausted only when all attempts were used.
+			if exhausted && err != nil {
+				err = fmt.Errorf("%w: %w", ErrRetryExhausted, err)
+			}
 			return resp, err
 		})
 	}