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

middleware/middleware.go

@@ -0,0 +1,29 @@
+package middleware
+
+import "net/http"
+
+// Middleware wraps an http.RoundTripper to add behavior.
+// This is the fundamental building block of the httpx library.
+type Middleware func(http.RoundTripper) http.RoundTripper
+
+// Chain composes middlewares so that Chain(A, B, C)(base) == A(B(C(base))).
+// Middlewares are applied from right to left: C wraps base first, then B wraps
+// the result, then A wraps last. This means A is the outermost layer and sees
+// every request first.
+func Chain(mws ...Middleware) Middleware {
+	return func(rt http.RoundTripper) http.RoundTripper {
+		for i := len(mws) - 1; i >= 0; i-- {
+			rt = mws[i](rt)
+		}
+		return rt
+	}
+}
+
+// RoundTripperFunc is an adapter to allow the use of ordinary functions as
+// http.RoundTripper. It works exactly like http.HandlerFunc for handlers.
+type RoundTripperFunc func(*http.Request) (*http.Response, error)
+
+// RoundTrip implements http.RoundTripper.
+func (f RoundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}