feat(review 008 Unit 3b): the hub-leg install template (ADR-051 §5)

- src/channels/hub_leg.rs: HubLegImports (the Clone discover/stash,
  bundles split by the channel_open marker, with the per-consumer
  filter) and HubLegTemplate (the hub-leg install hook: per-connection
  fork + generic channel ops + plain bundles as-is + marked specs via
  ChannelRelay::register_relay_openable + bootstrap discovery closed
  over the fork (review 004 F-06) + serving-identity resolution
  (CF-005) + the single-stream dispatch loop).
- The loud assembly posture holds at install: a Pub-typed (or
  unregistrable) marked spec ends the leg's install task — channel 0
  never dispatches, never a silent stub (ADR-051 §6).
- Assembly rule surfaced by the gate e2e: from_call discovers the
  spoke's bootstrap discovery ops like any plain op; the template
  skips BOOTSTRAP_DISCOVERY_OPS (new pub const, discovery.rs) when
  re-registering plain bundles — the template's own install, closed
  over the fork, supersedes the imported copies.
- Gate tests (7): stash split + filter, the full template e2e
  (services/list shows the re-exposed ops, a plain imported op
  round-trips through the hub, the marked op opens through the relay
  with bound surviving, data both directions), serving-identity
  precedence (scope-bearing override resolves, scope-less fails
  closed), the loud Pub-typed posture, and the filtered-stash subset
  run (NOT_FOUND for the filtered-out marked op).
- Docs: ADR-051 status (3a + 3b implemented, 3c remains); review 008
  Unit 3b landed note (the bootstrap-op skip rule).

Verification: cargo test (665 passed), clippy --all-targets
-D warnings, fmt --check, doc --no-deps — all clean.

src/channels/mod.rs

@@ -31,6 +31,10 @@
 //!   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.
+//! - [`hub_leg`]: `HubLegTemplate` — the hub-leg install template
+//!   (ADR-051 §5): fork + generic channel ops + bootstrap discovery +
+//!   plain bundles + relay openables + serving identity, composed into
+//!   the `install_channel_zero` hook every hub side needs.
 //! - [`client`]: `ChannelClient` — transport-agnostic
 //!   `from_connection` (ADR-043).
 //! - [`self::env`]: `ChannelOperationEnv` extension trait (ADR-047 §4 —
@@ -41,6 +45,7 @@
 pub mod adapter;
 pub mod client;
 pub mod env;
+pub mod hub_leg;
 pub mod manager;
 pub mod mux;
 pub mod operations;