docs(httputil): correct tier from 3 to 2

httputil depends on xerrors (Tier 0) and valid (Tier 1), placing it at
Tier 2. No infrastructure or lifecycle dependencies exist in this module.

doc.go

@@ -0,0 +1,11 @@
+// Package httputil provides typed HTTP handler adapters and response helpers.
+//
+// The centrepiece is [Handle], a generics-based adapter that wraps a pure Go
+// function into an [net/http.HandlerFunc], handling JSON decode, validation,
+// and error mapping automatically:
+//
+//	r.Get("/orders/{id}", httputil.Handle(validator, svc.GetOrder))
+//
+// Error responses are mapped from [xerrors.Code] to HTTP status codes.
+// All response helpers set Content-Type: application/json.
+package httputil