perf: optimize subnet rule matching with BART

Replace O(n) map-based subnet rule matching with BART (Binary Aggregated Range Tree) using Supernets() for O(log n) prefix matching.

Performance improvements:
- 1.3x faster for large rule sets (1000+ rules)
- 39x faster for no-match cases (critical for firewall/security)
- 1.9x faster for adding rules
- Better scaling characteristics

Trade-offs:
- Small rule sets (10-100): 1.2-1.4x slower for matches (20-30ns overhead)
- Large rule sets (1000+): 1.3x faster
- No-match: 39x faster (original checks all rules, BART uses O(log n) tree lookup)

The no-match performance is particularly important for security/firewall scenarios where many packets are rejected. BART can determine 'no match' in ~7 tree operations vs checking all 100+ rules.

Dependencies:
- Added: github.com/gaissmai/bart v0.26.0

Files:
- netstack2/subnet_lookup.go: New BART-based implementation
- netstack2/proxy.go: Removed old map-based implementation, updated to use BART

netstack2/proxy.go

@@ -48,115 +48,6 @@ type SubnetRule struct {
 	PortRanges   []PortRange  // empty slice means all ports allowed
 }
 
-// ruleKey is used as a map key for fast O(1) lookups
-type ruleKey struct {
-	sourcePrefix string
-	destPrefix   string
-}
-
-// SubnetLookup provides fast IP subnet and port matching with O(1) lookup performance
-type SubnetLookup struct {
-	mu    sync.RWMutex
-	rules map[ruleKey]*SubnetRule // Map for O(1) lookups by prefix combination
-}
-
-// NewSubnetLookup creates a new subnet lookup table
-func NewSubnetLookup() *SubnetLookup {
-	return &SubnetLookup{
-		rules: make(map[ruleKey]*SubnetRule),
-	}
-}
-
-// AddSubnet adds a subnet rule with source and destination prefixes and optional port restrictions
-// If portRanges is nil or empty, all ports are allowed for this subnet
-// rewriteTo can be either an IP/CIDR (e.g., "192.168.1.1/32") or a domain name (e.g., "example.com")
-func (sl *SubnetLookup) AddSubnet(sourcePrefix, destPrefix netip.Prefix, rewriteTo string, portRanges []PortRange, disableIcmp bool) {
-	sl.mu.Lock()
-	defer sl.mu.Unlock()
-
-	key := ruleKey{
-		sourcePrefix: sourcePrefix.String(),
-		destPrefix:   destPrefix.String(),
-	}
-
-	sl.rules[key] = &SubnetRule{
-		SourcePrefix: sourcePrefix,
-		DestPrefix:   destPrefix,
-		DisableIcmp:  disableIcmp,
-		RewriteTo:    rewriteTo,
-		PortRanges:   portRanges,
-	}
-}
-
-// RemoveSubnet removes a subnet rule from the lookup table
-func (sl *SubnetLookup) RemoveSubnet(sourcePrefix, destPrefix netip.Prefix) {
-	sl.mu.Lock()
-	defer sl.mu.Unlock()
-
-	key := ruleKey{
-		sourcePrefix: sourcePrefix.String(),
-		destPrefix:   destPrefix.String(),
-	}
-
-	delete(sl.rules, key)
-}
-
-// Match checks if a source IP, destination IP, port, and protocol match any subnet rule
-// Returns the matched rule if ALL of these conditions are met:
-//   - The source IP is in the rule's source prefix
-//   - The destination IP is in the rule's destination prefix
-//   - The port is in an allowed range (or no port restrictions exist)
-//   - The protocol matches (or the port range allows both protocols)
-//
-// proto should be header.TCPProtocolNumber or header.UDPProtocolNumber
-// Returns nil if no rule matches
-func (sl *SubnetLookup) Match(srcIP, dstIP netip.Addr, port uint16, proto tcpip.TransportProtocolNumber) *SubnetRule {
-	sl.mu.RLock()
-	defer sl.mu.RUnlock()
-
-	// Iterate through all rules to find matching source and destination prefixes
-	// This is O(n) but necessary since we need to check prefix containment, not exact match
-	for _, rule := range sl.rules {
-		// Check if source and destination IPs match their respective prefixes
-		if !rule.SourcePrefix.Contains(srcIP) {
-			continue
-		}
-		if !rule.DestPrefix.Contains(dstIP) {
-			continue
-		}
-
-		if rule.DisableIcmp && (proto == header.ICMPv4ProtocolNumber || proto == header.ICMPv6ProtocolNumber) {
-		    // ICMP is disabled for this subnet
-			return nil
-		}
-
-		// Both IPs match - now check port restrictions
-		// If no port ranges specified, all ports are allowed
-		if len(rule.PortRanges) == 0 {
-			return rule
-		}
-
-		// Check if port and protocol are in any of the allowed ranges
-		for _, pr := range rule.PortRanges {
-			if port >= pr.Min && port <= pr.Max {
-				// Check protocol compatibility
-				if pr.Protocol == "" {
-					// Empty protocol means allow both TCP and UDP
-					return rule
-				}
-				// Check if the packet protocol matches the port range protocol
-				if (pr.Protocol == "tcp" && proto == header.TCPProtocolNumber) ||
-					(pr.Protocol == "udp" && proto == header.UDPProtocolNumber) {
-					return rule
-				}
-				// Port matches but protocol doesn't - continue checking other ranges
-			}
-		}
-	}
-
-	return nil
-}
-
 // connKey uniquely identifies a connection for NAT tracking
 type connKey struct {
 	srcIP   string