feat(httputil): add Bind/BindEmpty request binding from path and query; v1.6.0

Bind and BindEmpty fill Req from path/query/json struct tags and validate
once, extending the typed decode->validate->call->encode pipeline to routes
with identifiers and filters. Conversion via builtins + encoding.TextUnmarshaler
(uuid.UUID, time.Time); malformed value -> 400 naming the parameter; default:
applies only when absent; repeated query -> slice; mis-tagged struct panics at
wiring. Purely additive; existing adapters unchanged. Coordinated lockstep v1.6.0.

httputil/handler_func.go

@@ -6,7 +6,13 @@ var _ http.Handler = HandlerFunc(nil)
 
 // HandlerFunc is an http.Handler that returns an error.
 // On non-nil error the error is mapped to the appropriate HTTP response via [Error].
-// Use for manual handlers that need path parameters or custom status codes.
+//
+// Use it for genuinely custom responses — streaming, file downloads, non-JSON
+// content types — where the typed adapters do not fit. It is no longer the answer
+// for path or query parameters: [Bind] and [BindEmpty] fill those from struct tags
+// with the same decode → validate → encode guarantees, and a custom success status
+// is set with [WithStatus]. Reaching for HandlerFunc to read a parameter is the one
+// path by which a handler reaches production without validation running.
 type HandlerFunc func(w http.ResponseWriter, r *http.Request) error
 
 // ServeHTTP implements http.Handler.