docs(handler/masque): add comprehensive comments for MASQUE proxy handler

Add detailed documentation to the masque handler package covering:
- Package-level doc explaining MASQUE protocol (RFC 9298 CONNECT-UDP,
  RFC 9114 CONNECT-TCP) and request flow
- Doc comments on all exported and unexported types, methods, and structs
- Inline comments explaining HTTP/3 context extraction and data path
- Field-level comments on metadata struct explaining each config param
- Step-by-step flow and data path diagrams for handleConnectUDP and
  handleConnectTCP

handler/masque/metadata.go

@@ -8,22 +8,44 @@ import (
 )
 
 const (
+	// defaultBufferSize is the default UDP relay buffer size in bytes.
 	defaultBufferSize = 4096
-	defaultRealm      = "gost"
+	// defaultRealm is the default HTTP Basic authentication realm for proxy auth.
+	defaultRealm = "gost"
 )
 
+// metadata holds parsed configuration values for the MASQUE handler.
+// Values are extracted from the generic metadata map via parseMetadata.
 type metadata struct {
-	hash           string
-	bufferSize     int
-	authBasicRealm          string
-	idleTimeout             time.Duration
+	// hash defines the hash source for load-balancing selector.
+	// When set to "host", the target address is used as the hash input
+	// for consistent routing through chain hops.
+	hash string
+	// bufferSize is the UDP relay buffer size in bytes for datagram forwarding.
+	bufferSize int
+	// authBasicRealm is the realm string sent in Proxy-Authenticate challenges.
+	authBasicRealm string
+	// idleTimeout is the read timeout for TCP connections during bidirectional relay.
+	// After this duration with no data, the relay is terminated.
+	idleTimeout time.Duration
 
-	observerPeriod         time.Duration
-	observerResetTraffic   bool
+	// observerPeriod is the interval between observer stats collection cycles.
+	observerPeriod time.Duration
+	// observerResetTraffic controls whether per-client traffic counters are
+	// reset to zero after each observer report.
+	observerResetTraffic bool
+	// limiterRefreshInterval controls how often the cached traffic limiter
+	// refreshes its limits from the global limiter source.
 	limiterRefreshInterval time.Duration
+	// limiterCleanupInterval controls how often the cached traffic limiter
+	// removes stale cached entries for disconnected clients.
 	limiterCleanupInterval time.Duration
 }
 
+// parseMetadata extracts typed configuration values from the generic metadata map.
+// Metadata keys support multiple fallback names for compatibility (e.g., "bufferSize"
+// and "udp.bufferSize"). Duration values accept both integer (seconds) and string
+// (Go duration format like "5s") inputs.
 func (h *masqueHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.hash = mdutil.GetString(md, "hash")