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

error.go

@@ -1,16 +1,21 @@
 package httpx
 
 import (
-	"errors"
 	"fmt"
 	"net/http"
+
+	"git.codelab.vc/pkg/httpx/balancer"
+	"git.codelab.vc/pkg/httpx/circuitbreaker"
+	"git.codelab.vc/pkg/httpx/retry"
 )
 
 // Sentinel errors returned by httpx components.
+// These are aliases for the canonical errors defined in sub-packages,
+// so that errors.Is works regardless of which import the caller uses.
 var (
-	ErrRetryExhausted = errors.New("httpx: all retry attempts exhausted")
-	ErrCircuitOpen    = errors.New("httpx: circuit breaker is open")
-	ErrNoHealthy      = errors.New("httpx: no healthy endpoints available")
+	ErrRetryExhausted = retry.ErrRetryExhausted
+	ErrCircuitOpen    = circuitbreaker.ErrCircuitOpen
+	ErrNoHealthy      = balancer.ErrNoHealthy
 )
 
 // Error provides structured error information for failed HTTP operations.