feat(03-03): add typed path params and per-plugin rollback

Compile regex and enum constraints at route registration so malformed and unknown IDs share a 404, and named rollback errors isolate one plugin's history.

Co-authored-by: Cursor <cursoragent@cursor.com>

examples/hello/plugins/greeter/plugin.go

@@ -59,13 +59,19 @@ func (p *Plugin) Boot(app *backpack.App) error {
 func (p *Plugin) Routes(r pact.Router) error {
 	r.Get("/items/{id}", func(w http.ResponseWriter, req *http.Request) {
 		id, ok := surf.IntParam(req, "id")
-		if !ok {
+		if !ok || id != 1 {
 			http.NotFound(w, req)
 			return
 		}
 		w.Header().Set("Content-Type", "application/json")
 		fmt.Fprintf(w, `{"id":%d}`, id)
 	})
+	r.Where("id", `[0-9]+`)
+	r.Get("/kinds/{kind}", func(w http.ResponseWriter, req *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprintf(w, `{"kind":%q}`, req.PathValue("kind"))
+	})
+	r.WhereIn("kind", "widget", "gadget")
 	return nil
 }