Improves flow handling and adds runtime stats APIs

Refactors TCP and UDP flow managers to enhance analyzer selection and flow binding accuracy, including O(1) UDP stream rebinding by 5-tuple.
Introduces runtime stats tracking for engine and ruleset operations, exposing new APIs for granular performance and error metrics.
Optimizes GeoMatcher with result caching and supports efficient geosite set matching, reducing redundant computation in ruleset expressions.

engine/analyzer_selector_test.go

@@ -0,0 +1,56 @@
+package engine
+
+import (
+	"testing"
+
+	"git.difuse.io/Difuse/Mellaris/analyzer"
+)
+
+type namedAnalyzer struct{ name string }
+
+func (a namedAnalyzer) Name() string { return a.name }
+func (a namedAnalyzer) Limit() int   { return 0 }
+
+func TestSignatureSelectorTCPPrunesByPayloadNotPort(t *testing.T) {
+	sel := newAnalyzerSelector(AnalyzerSelectionModeSignature, &statsCounters{})
+	all := []analyzer.Analyzer{
+		namedAnalyzer{"http"},
+		namedAnalyzer{"tls"},
+		namedAnalyzer{"trojan"},
+		namedAnalyzer{"ssh"},
+		namedAnalyzer{"socks"},
+		namedAnalyzer{"fet"},
+	}
+	// TLS record-like prefix, regardless of destination port.
+	payload := []byte{0x16, 0x03, 0x03, 0x00, 0x10}
+	selected := sel.SelectTCP(all, payload)
+
+	got := make(map[string]bool)
+	for _, a := range selected {
+		got[a.Name()] = true
+	}
+	for _, keep := range []string{"tls", "trojan", "fet"} {
+		if !got[keep] {
+			t.Fatalf("expected analyzer %q to be selected", keep)
+		}
+	}
+	for _, drop := range []string{"http", "ssh", "socks"} {
+		if got[drop] {
+			t.Fatalf("expected analyzer %q to be pruned", drop)
+		}
+	}
+}
+
+func TestSignatureSelectorConservativeFallback(t *testing.T) {
+	sel := newAnalyzerSelector(AnalyzerSelectionModeSignature, &statsCounters{})
+	all := []analyzer.Analyzer{
+		namedAnalyzer{"http"},
+		namedAnalyzer{"tls"},
+		namedAnalyzer{"custom"},
+	}
+	payload := []byte{0xde, 0xad, 0xbe, 0xef}
+	selected := sel.SelectTCP(all, payload)
+	if len(selected) != len(all) {
+		t.Fatalf("expected conservative fallback to keep all analyzers, got=%d want=%d", len(selected), len(all))
+	}
+}