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
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