docs(review 008 Unit 3 planning): ADR-051 — in-tree ChannelRelay + hub-leg assembly; plan split into 3a/3b/3c

Design session outcome for the relay unit. The hub/spoke family (hub
or spoke may relay; the hub re-exposes spoke services by ACL without
binding ports) needs the call-half support too, so the unit's scope
grew beyond the as-pinned sketch and is now three sub-units.

- ADR-051 (new): the relay is the wrapper composition — translate hop
  = the ADR-049 establisher shape, byte-forward hop = pump_bidi per
  ADR-050; the relay holds Arc<CallConnection> + ChannelManager per
  producer leg (not a ChannelClient — take_call_connection's detach
  is wrong for a hub with three CallConnection claimants); the reason
  mapping preserves the spoke's code+message (timeout is the one
  non-1:1 case, mapping to dial_failed); the registration seam is
  two-phase (discover/stash → per-connection fork-register — the
  ADR-047 §4 fork mechanism makes a one-call import impossible); the
  ADR-042 relay map dissolves (implicit per-channel mapping) and
  channel/close needs no translation surface (EOF cascade propagates
  with correct ledger accounting on both legs); Pub-typed marked
  specs are a loud assembly error; establishment bounds compound per
  hop (noted, no fix); the ACL layering note is pinned (the spoke's
  AccessControl sees only the hub identity; forwarded_for is never
  checked).
- ADR-042 amended: the §Scope note is revised (implementation is an
  alkcall export; hub crates compose it) and the two mechanism
  supersessions are recorded — the contract and auth-model rationale
  unchanged.
- ADR-047 amendment 3's forward reference and the README ADR index
  updated (001..051).
- Review 008 remediation plan: Unit 3 split into 3a (ChannelRelay
  component + gates), 3b (hub-leg install template — the call-half
  support), 3c (gate-2 e2e incl. the mid-establishment disconnect
  window); sequencing note updated; adoption note records the
  session's decisions.

Verified: cargo doc --no-deps (markdown-only change).

docs/architecture/decisions/047-openable-alpns-are-operations.md

@@ -98,7 +98,8 @@ multi-segment `channels/custom/proto/sub`) round-trip byte-stable
 string without the boolean never marks; `op/register` announced
 flavor-form specs round-trip with the marker. Gate 2 (the hub-relay
 round trip) lands with the in-tree relay component (review 008
-remediation plan Unit 3, amending ADR-042).
+remediation plan Unit 3c, amending ADR-042 — the relay export is
+ADR-051).
 
 ## Amendment (§6 listing enrichment, 2026-09-06)