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Breaking wire format change: DataPacket payloads inside the AEAD envelope now carry a 4-byte port header [src_port:2 LE][dst_port:2 LE] before the service payload. The receiver dispatches by destination port. Port multiplexing: - send_session_data() takes src_port/dst_port params, prepends port header - New send_ipv6_packet() compresses IPv6 header and sends on port 256 - Receive path dispatches DataPackets by port: port 256 decompresses IPv6 header from session context and delivers to TUN, unknown ports dropped - Port constants: FSP_PORT_HEADER_SIZE (4 bytes), FSP_PORT_IPV6_SHIM (256) IPv6 header compression: - New ipv6_shim module with compress_ipv6()/decompress_ipv6() pure functions - Strips src/dst addresses (32 bytes) and payload length (2 bytes) from each packet, preserving traffic class, flow label, next header, and hop limit as 6-byte residual fields - Addresses reconstructed from session context on receive side - Net savings: 29 bytes per packet (overhead 106 → 77 bytes) - FIPS_IPV6_OVERHEAD constant (77 bytes), effective_ipv6_mtu() updated - 16 unit tests for round-trip fidelity, field preservation, error cases Documentation: - fips-wire-formats: DataPacket port header, port registry, IPv6 shim format tables, updated encapsulation walkthrough and overhead budget - fips-ipv6-adapter: FIPS_IPV6_OVERHEAD (77 bytes), updated MTU numbers, TUN reader/writer flow with compression steps, impl status - fips-session-layer: port-based service dispatch section, data transfer description, impl status - fips-intro: IPv6 adapter as port 256 service, node architecture updated - fips-mesh-operation: packet size summary with compressed overhead - DataPacket doc updated with port header and dispatch model - session_wire.rs module doc: DataPacket Port Multiplexing section