fix: reduce per-request context and fallback overhead

Make Context keys lazy so requests that never call Set stop allocating on reset. Reuse stable 404 and 405 handlers and add focused benchmarks so ServeHTTP miss paths stay measurable.

engine_benchmark_test.go

@@ -0,0 +1,64 @@
+package touka
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+var benchmarkStatusCode int
+
+func buildServeHTTPBenchmarkEngine() *Engine {
+	engine := New()
+	engine.GET("/api/v1/users", func(c *Context) {
+		c.Status(http.StatusNoContent)
+	})
+	engine.GET("/api/v1/users/:id", func(c *Context) {
+		c.Status(http.StatusNoContent)
+	})
+	engine.POST("/api/v1/users", func(c *Context) {
+		c.Status(http.StatusNoContent)
+	})
+	return engine
+}
+
+func benchmarkServeHTTP(b *testing.B, engine *Engine, method, path string) {
+	b.Helper()
+
+	req, err := http.NewRequest(method, path, nil)
+	if err != nil {
+		b.Fatalf("failed to build request: %v", err)
+	}
+	rr := httptest.NewRecorder()
+	engine.ServeHTTP(rr, req)
+
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		rr = httptest.NewRecorder()
+		engine.ServeHTTP(rr, req)
+	}
+
+	benchmarkStatusCode = rr.Code
+}
+
+func BenchmarkServeHTTP(b *testing.B) {
+	engine := buildServeHTTPBenchmarkEngine()
+
+	b.Run("StaticHit", func(b *testing.B) {
+		benchmarkServeHTTP(b, engine, http.MethodGet, "/api/v1/users")
+	})
+
+	b.Run("NotFound", func(b *testing.B) {
+		benchmarkServeHTTP(b, engine, http.MethodGet, "/does/not/exist")
+	})
+
+	b.Run("MethodNotAllowed", func(b *testing.B) {
+		benchmarkServeHTTP(b, engine, http.MethodDelete, "/api/v1/users")
+	})
+
+	b.Run("OptionsAllow", func(b *testing.B) {
+		benchmarkServeHTTP(b, engine, http.MethodOptions, "/api/v1/users")
+	})
+}