feat(review 007 Unit 3): pump_bidi two-pump helper (R-03, ADR-050)

Extracts the two-pump data-plane helper alknet ADR-078 deferred until
the shapes converged (they have: alktunnels POC pump_halves + alktty's
channels session). Purely additive.

- channels::pump::pump_bidi(channel, peer_read, peer_write) -> (u64, u64):
  two joined pumps, shutdown-on-completion per direction; copy counts
  for observability. The channel side is a single AsyncRead +
  AsyncWrite value (the accept_bi BiStream); the peer side takes split
  halves — the establisher's natural dial result (into_split).
- Return (u64, u64), not the review sketch's io::Result<(u64, u64)>:
  both pumps swallow copy errors by contract (mid-stream error =
  abrupt close, no error channel mid-stream per ADR-049 §6), so an
  Err state would be dead code. Deviation recorded in ADR-050.
- alktty's three-pump session does not fit (exit future as a third
  signal) and stays as-is, per the review's scope.
- Tests reproduce the POC's two-pump semantics through the helper:
  bidirectional flow with exact counts, EOF-from-one-side completes
  the other's shutdown (clean EOF at the far end), dead-source =
  EOF-shaped teardown.
- ADR-050 records the decision, deviations, and two-way door type.

Verification: cargo test (625 passed, +2), clippy -D warnings, fmt
--check, doc clean, test --all-features clean, wasm32-unknown-unknown
check clean.

src/channels/mod.rs

@@ -25,6 +25,8 @@
 //!   ALPN crates via `ChannelCore` (ADR-047 §3).
 //! - [`policy`]: `ChannelLifecyclePolicy` + `PerIdentityChannelPolicy`
 //!   (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).
 //! - [`client`]: `ChannelClient` — transport-agnostic
 //!   `from_connection` (ADR-043).
 //! - [`self::env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
@@ -39,6 +41,7 @@ pub mod manager;
 pub mod mux;
 pub mod operations;
 pub mod policy;
+pub mod pump;
 pub mod reassembly;
 pub mod source;
 pub mod wire;