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

go.mod

@@ -0,0 +1,3 @@
+module git.codelab.vc/pkg/httpx
+
+go 1.24