feat: release configurable API response contracts

Define reusable response bodies in .api with typed code mappings, per-route overrides and matching OpenAPI schemas. Add application and callback encoders, early response policy selection, and isolated remote-consumer release verification.

Release v0.8.0-preview after full race, vet, build and local MySQL workflow acceptance.

transport/nethttp/router.go

@@ -11,7 +11,10 @@ import (
 	"git.yuntuo.hk/k2safe/go-omni/transport/internal/routing"
 )
 
-type Router struct{ routes []route }
+type Router struct {
+	routes                     []route
+	notFound, methodNotAllowed http.Handler
+}
 
 type route struct {
 	method   string
@@ -22,6 +25,12 @@ type route struct {
 
 func New() *Router { return &Router{} }
 
+// SetFallbacks must be called before serving requests. Nil preserves the plain
+// net/http fallback, useful when using this router without an Omni application.
+func (r *Router) SetFallbacks(notFound, methodNotAllowed http.Handler) {
+	r.notFound, r.methodNotAllowed = notFound, methodNotAllowed
+}
+
 func (r *Router) ServeHTTP(writer http.ResponseWriter, request *http.Request) {
 	var selected *route
 	var selectedParams map[string]string
@@ -48,9 +57,17 @@ func (r *Router) ServeHTTP(writer http.ResponseWriter, request *http.Request) {
 			}
 			sort.Strings(methods)
 			writer.Header().Set("Allow", strings.Join(methods, ", "))
-			http.Error(writer, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
+			if r.methodNotAllowed != nil {
+				r.methodNotAllowed.ServeHTTP(writer, request)
+			} else {
+				http.Error(writer, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
+			}
 		} else {
-			http.NotFound(writer, request)
+			if r.notFound != nil {
+				r.notFound.ServeHTTP(writer, request)
+			} else {
+				http.NotFound(writer, request)
+			}
 		}
 		return
 	}