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.

context_benchmark_test.go

@@ -0,0 +1,78 @@
+package touka
+
+import (
+	"net/http"
+	"testing"
+)
+
+func TestContextResetKeepsKeysNilUntilSet(t *testing.T) {
+	c, _ := CreateTestContext(nil)
+	if c.Keys != nil {
+		t.Fatalf("expected fresh test context Keys to be nil before first Set")
+	}
+
+	c.Set("answer", 42)
+	if c.Keys == nil {
+		t.Fatalf("expected Set to allocate Keys map")
+	}
+	if value, exists := c.Get("answer"); !exists || value != 42 {
+		t.Fatalf("expected stored value to round-trip, got %v, %t", value, exists)
+	}
+
+	req, err := http.NewRequest(http.MethodGet, "/", nil)
+	if err != nil {
+		t.Fatalf("failed to build request: %v", err)
+	}
+	c.reset(c.Writer, req)
+
+	if c.Keys != nil {
+		t.Fatalf("expected reset to clear Keys without allocating a new map")
+	}
+	if value, exists := c.Get("answer"); exists || value != nil {
+		t.Fatalf("expected cleared keys after reset, got %v, %t", value, exists)
+	}
+
+	ctxValue := c.Value("missing")
+	if ctxValue != nil {
+		t.Fatalf("expected nil value for missing context key after reset, got %v", ctxValue)
+	}
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatalf("expected MustGet to panic for missing key after reset")
+		}
+	}()
+	_ = c.MustGet("answer")
+}
+
+func BenchmarkContextReset(b *testing.B) {
+	b.Run("NoKeysUse", func(b *testing.B) {
+		c, _ := CreateTestContext(nil)
+		req, err := http.NewRequest(http.MethodGet, "/", nil)
+		if err != nil {
+			b.Fatalf("failed to build request: %v", err)
+		}
+
+		b.ReportAllocs()
+		b.ResetTimer()
+
+		for i := 0; i < b.N; i++ {
+			c.reset(c.Writer, req)
+		}
+	})
+
+	b.Run("WithKeysUse", func(b *testing.B) {
+		c, _ := CreateTestContext(nil)
+		req, err := http.NewRequest(http.MethodGet, "/", nil)
+		if err != nil {
+			b.Fatalf("failed to build request: %v", err)
+		}
+
+		b.ReportAllocs()
+		b.ResetTimer()
+
+		for i := 0; i < b.N; i++ {
+			c.reset(c.Writer, req)
+			c.Set("request-id", i)
+		}
+	})
+}