feat(19.7-01): in-process dispatch marker and compiled handler lookup

- bouncer.Dispatch, WithDispatch, Dispatched: context-only pre-resolved
  principal marker honoured by Registry.Middleware for listed guard names
- cabana.ResolveBackend honours a backend-kind marker; additive
  ResolveBackendToken, ScopeLimitFor and BackendPrincipal resolvers
- surf.Router.Handler and compileHandlers: one compiled chain per route,
  shared by BuildRouter validation and compile
- smoke tests TestDispatchSmoke, TestDispatchedSmoke, TestDispatchMarkerSmoke
- READMEs and docs/services/{authentication,routing}.md updated

modules/surf/routetable.go

@@ -1,5 +1,7 @@
 package surf
 
+import "net/http"
+
 // RouteInfo is a read-only snapshot of one registered route after Assemble
 // or BuildRouter.
 type RouteInfo struct {
@@ -37,3 +39,17 @@ func (r *Router) Routes() []RouteInfo {
 	}
 	return out
 }
+
+// Handler returns the compiled middleware chain of the route registered for
+// method and pattern (as listed by Routes), for an in-process dispatch that
+// must run the same constraints, guards, throttles, body limit, locale and
+// recovery as an HTTP request. The caller sets the path values and Pattern
+// on its request before serving, as the mux would. False when no such route
+// exists or BuildRouter has not compiled the routes yet.
+func (r *Router) Handler(method, pattern string) (http.Handler, bool) {
+	if r == nil || r.handlers == nil {
+		return nil, false
+	}
+	h, ok := r.handlers[method+" "+pattern]
+	return h, ok && h != nil
+}