feat(uring): carry ECN on the io_uring UDP path

[kixelated]

Summary

Completes and deletes quest/m2/quic/ecn-uring.md; ecn-measure.md becomes ready and ecn.md (L4S) drops the blocker.

rs/moq-uring/src/udp.rs carried only UDP_SEGMENT on send and UDP_GRO on receive, so a relay on the io_uring runtime never marked ECT(0) and never read a mark back. The peer's ACKs carried no ECN counts and noq-proto disabled ECN on the first one. noq-udp already did this right; this brings the ring's path level with it.

• udp::Ecn (Ect0/Ect1/Ce, same bits as noq's EcnCodepoint) and udp::Transmit { to, len, segment, ecn }; TxBuf::send takes the struct instead of growing to four arguments.
• Send writes IP_TOS (v4 or v4-mapped destination) or IPV6_TCLASS beside UDP_SEGMENT in the same control buffer, so a GSO train marks every segment.
• Bind enables IP_RECVTOS and, on a v6 socket, IPV6_RECVTCLASS too (Linux reports a v4-mapped datagram's mark as IP_TOS). Failure is an error, not a warning.
• gro_stride becomes a RecvMeta walk over every control message; Packet::ecn() reports the mark (GRO only coalesces datagrams with the same TOS, so one per completion is exact).
• The noq and quinn adapter passes Transmit::ecn at both send sites and Packet::ecn() into both Endpoint::handle calls. quiche has no ECN API; its callers send unmarked.

Regression

rs/moq-uring/tests/ecn.rs: a worker socket pair round-trips ECT(0) over v4 and v6 as a lone datagram, a GSO train, and with GRO/GSO/multishot off; CE survives; an unmarked send reads back None. Fails on main (no cmsg either way). Runs in the existing rs uring nightly lane and skips loudly below the kernel floor like its siblings.

Verification

cargo clippy -p moq-uring --lib --target x86_64-unknown-linux-gnu -D warnings passes cross-compiled from macOS for the noq, quinn, and qlog feature sets. The test targets cannot be cross-checked from macOS (boring-sys bindgen needs Linux libc headers), so the tests have not run yet; they need a Linux 6.12+ box: just rs uring.

Public API and wire impact

moq-uring is 0.0.1 and unpublished. udp::TxBuf::send signature changes, udp::Ecn, udp::Transmit, and udp::Packet::ecn are new. No wire, config, or doc change; a relay's packets now carry ECT(0) on the io_uring runtime, as they already did on tokio.

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(written by Claude Opus 5)