Add process information cache to avoid duplicate lookups

PreMatch and full match phases each created a fresh InboundContext,
causing process search (expensive OS syscalls) to run twice per
connection. Use a freelru ShardedLRU cache with 200ms TTL to serve
the second lookup from cache.

route/process_cache.go

@@ -0,0 +1,34 @@
+package route
+
+import (
+	"context"
+	"net/netip"
+
+	"github.com/sagernet/sing-box/adapter"
+	"github.com/sagernet/sing-box/common/process"
+)
+
+type processCacheKey struct {
+	Network     string
+	Source      netip.AddrPort
+	Destination netip.AddrPort
+}
+
+type processCacheEntry struct {
+	result *adapter.ConnectionOwner
+	err    error
+}
+
+func (r *Router) findProcessInfoCached(ctx context.Context, network string, source netip.AddrPort, destination netip.AddrPort) (*adapter.ConnectionOwner, error) {
+	key := processCacheKey{
+		Network:     network,
+		Source:      source,
+		Destination: destination,
+	}
+	if entry, ok := r.processCache.Get(key); ok {
+		return entry.result, entry.err
+	}
+	result, err := process.FindProcessInfo(r.processSearcher, ctx, network, source, destination)
+	r.processCache.Add(key, processCacheEntry{result: result, err: err})
+	return result, err
+}