v0.7.3-beta2: single-erasure FEC, fully fixed and verified end-to-end Includes the complete single-erasure FEC feature built and hardened this session: - Opt-in scheduler.fec_enabled/fec_group_size XOR parity for real Data traffic (wire format, sender/receiver integration, diagnostics). - MTU accounting fix so parity frames can never exceed the tunnel MTU. - fec_enabled=false confirmed zero-cost on the receive hot path. - Duplicate-delivery bug closed with a seen-seq floor watermark (formally proven + empirically verified via a reverted probe test). - Phantom-group-member bug closed: FEC accumulation is now gated on confirmed real transmission, propagated from WgTunnel::encapsulate through wg_send through send_scheduled -- previously a silently dropped or WireGuard-handshake-substituted frame could poison a group's recovery budget or cause FEC to reconstruct and deliver a packet that was never actually sent by the peer. - Real tests/veth_fec.rs integration tests, executed for real (not just unit-tested) on a live TUN-capable host: 3/3 passing. Kept on the 0.7.3 version line per explicit direction -- this is a beta iteration, not a new Cargo.toml version bump.