Add foundation: middleware type, error types, and internal clock

Introduce the core building blocks for the httpx library:
- middleware.Middleware type and Chain() composer
- Error struct with sentinel errors (ErrRetryExhausted, ErrCircuitOpen, ErrNoHealthy)
- internal/clock package with Clock interface and MockClock for deterministic testing

error.go

@@ -0,0 +1,44 @@
+package httpx
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+)
+
+// Sentinel errors returned by httpx components.
+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")
+)
+
+// Error provides structured error information for failed HTTP operations.
+type Error struct {
+	// Op is the operation that failed (e.g. "Get", "Do").
+	Op string
+	// URL is the originally-requested URL.
+	URL string
+	// Endpoint is the resolved endpoint URL (after balancing).
+	Endpoint string
+	// StatusCode is the HTTP status code, if a response was received.
+	StatusCode int
+	// Retries is the number of retry attempts made.
+	Retries int
+	// Err is the underlying error.
+	Err error
+}
+
+func (e *Error) Error() string {
+	if e.Endpoint != "" && e.Endpoint != e.URL {
+		return fmt.Sprintf("httpx: %s %s (endpoint %s): %v", e.Op, e.URL, e.Endpoint, e.Err)
+	}
+	return fmt.Sprintf("httpx: %s %s: %v", e.Op, e.URL, e.Err)
+}
+
+func (e *Error) Unwrap() error { return e.Err }
+
+// ErrorMapper maps an HTTP response to an error. If the response is
+// acceptable, the mapper should return nil. Used by Client to convert
+// non-successful HTTP responses into Go errors.
+type ErrorMapper func(resp *http.Response) error