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/interface.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"context"
+	"sync/atomic"
 
 	"git.difuse.io/Difuse/Mellaris/io"
 	"git.difuse.io/Difuse/Mellaris/ruleset"
@@ -13,6 +14,49 @@ type Engine interface {
 	UpdateRuleset(ruleset.Ruleset) error
 	// Run runs the engine, until an error occurs or the context is cancelled.
 	Run(context.Context) error
+	// Stats returns a consistent snapshot of runtime counters.
+	Stats() Stats
+}
+
+type OverflowPolicy string
+
+const (
+	OverflowPolicyAccept       OverflowPolicy = "accept"
+	OverflowPolicyDrop         OverflowPolicy = "drop"
+	OverflowPolicyBackpressure OverflowPolicy = "backpressure"
+)
+
+type AnalyzerSelectionMode string
+
+const (
+	AnalyzerSelectionModeAlways    AnalyzerSelectionMode = "always"
+	AnalyzerSelectionModeSignature AnalyzerSelectionMode = "signature"
+)
+
+type statsCounters struct {
+	OverflowEvents             atomic.Uint64
+	OverflowAccepts            atomic.Uint64
+	OverflowDrops              atomic.Uint64
+	OverflowBackpressureEvents atomic.Uint64
+
+	AnalyzerSelectionsTotal  atomic.Uint64
+	AnalyzerSelectionsPruned atomic.Uint64
+
+	UDPTupleLookups atomic.Uint64
+	UDPTupleHits    atomic.Uint64
+}
+
+type Stats struct {
+	OverflowEvents             uint64
+	OverflowAccepts            uint64
+	OverflowDrops              uint64
+	OverflowBackpressureEvents uint64
+
+	AnalyzerSelectionsTotal  uint64
+	AnalyzerSelectionsPruned uint64
+
+	UDPTupleLookups uint64
+	UDPTupleHits    uint64
 }
 
 // Config is the configuration for the engine.
@@ -26,6 +70,8 @@ type Config struct {
 	WorkerTCPMaxBufferedPagesTotal   int
 	WorkerTCPMaxBufferedPagesPerConn int
 	WorkerUDPMaxStreams              int
+	OverflowPolicy                   OverflowPolicy
+	AnalyzerSelectionMode            AnalyzerSelectionMode
 }
 
 // Logger is the combined logging interface for the engine, workers and analyzers.