fix: cut redirect and allow-path routing overhead

Reuse fixed-path and Allow-header buffers so redirect and OPTIONS handling stop rebuilding temporary data on every request. Cache fallback chains and add regression coverage for redirect, 404, 405, and Allow behavior to keep the faster miss paths stable.

engine_benchmark_test.go

@@ -16,6 +16,9 @@ func buildServeHTTPBenchmarkEngine() *Engine {
 	engine.GET("/api/v1/users/:id", func(c *Context) {
 		c.Status(http.StatusNoContent)
 	})
+	engine.GET("/api/v1/users/:id/settings", func(c *Context) {
+		c.Status(http.StatusNoContent)
+	})
 	engine.POST("/api/v1/users", func(c *Context) {
 		c.Status(http.StatusNoContent)
 	})
@@ -61,4 +64,8 @@ func BenchmarkServeHTTP(b *testing.B) {
 	b.Run("OptionsAllow", func(b *testing.B) {
 		benchmarkServeHTTP(b, engine, http.MethodOptions, "/api/v1/users")
 	})
+
+	b.Run("FixedPathRedirect", func(b *testing.B) {
+		benchmarkServeHTTP(b, engine, http.MethodGet, "/API/V1/USERS/123/SETTINGS")
+	})
 }