- Return errors from MasqueConn.Read/Write instead of silently
succeeding (prevents infinite busy-loops and silent data loss
if these methods are accidentally called).
- Set recorder Network field in protocol dispatch switch rather
than hardcoding "udp", so early errors don't misreport type.
- Clarify that DatagramConn context-ID handling only supports
context ID 0 (sufficient for CONNECT-UDP per RFC 9298).
Implement UDP tunneling over HTTP/3 using HTTP Datagrams (RFC 9297):
- Add masque handler for server-side CONNECT-UDP
- Add masque connector and h3-masque dialer for client-side
- Add enableDatagrams option to HTTP/3 listener
- Add shared utilities for datagram connections and path parsing
Resource management and connection caching:
- Add deferred stream cleanup in connector on error paths
- Add IsClosed() and Close() methods to Client for proper session management
- Clean up stale cached clients in dialer before reuse
- Close underlying stream when DatagramConn is closed
- Move RequestStream opening from connector to dialer to enable dead
connection detection and cache invalidation (follows QUIC dialer pattern)