feat(review 008 Unit 3a): in-tree ChannelRelay (ADR-051, ADR-042 amendment)

- src/channels/relay.rs: ChannelRelay::register_relay_openable —
  registers a from_call-imported marked spec on a consumer-leg
  channel-0 registry via the full open-op wrapper (translate hop =
  the ADR-049 establisher calling the producer leg with the
  forwarded payload; byte-forward hop = pump_bidi, awaited inline).
  The relay map dissolves (implicit per-channel mapping; ids are
  per-connection), the spoke reply's channel_id is stripped, the
  reply's other fields ride with_reply_fields (bound flows
  end-to-end), reason mapping per ADR-051 §3 (spoke code+message
  preserved; timeout → dial_failed).
- Rejection postures (ADR-051 §6): unmarked and Pub-typed marked
  specs are loud assembly errors.
- Gate tests (9): registration rejections, reason mapping,
  forwarded_for payload shape, the real-wire consumer → hub → spoke
  round trip (bound survives, data both directions, the hub never
  parses the data plane), spoke-failure teardown (ledger
  decremented, no spoke channel leak), and a channels/tty/sub
  standard-shape run pinning no regression.

Verification: cargo test (658 passed), clippy --all-targets
-D warnings, fmt --check, doc --no-deps — all clean.

src/channels/mod.rs

@@ -27,6 +27,10 @@
 //!   (ADR-041, amended by ADR-047 §7 — opener ledger).
 //! - [`pump`]: `pump_bidi` — the two-pump data-plane helper (alknet
 //!   ADR-078, pinned upstream by ADR-050).
+//! - [`relay`]: `ChannelRelay` — the in-tree relay component (ADR-042
+//!   as amended by ADR-051): translate hop + `pump_bidi` byte-forward
+//!   hop for from_call-imported marked specs re-exposed on a hub's
+//!   consumer-leg channel-0 registry.
 //! - [`client`]: `ChannelClient` — transport-agnostic
 //!   `from_connection` (ADR-043).
 //! - [`self::env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
@@ -43,5 +47,6 @@ pub mod operations;
 pub mod policy;
 pub mod pump;
 pub mod reassembly;
+pub mod relay;
 pub mod source;
 pub mod wire;