docs(arch): rewrite hub README for multi-transport + channels substrate; add OQ-58 (worker registration)

The hub README predates channels (2026-07-09) and was built on the
call-protocol-directly-over-QUIC model: 'One QUIC connection per peer
carrying the call protocol,' CallClient::connect as the dial path,
QUIC as the only transport. The channels crate replaced that substrate
(ADR-071/079/080), and the hub's primary use case (worker provisioning
on docker/vast.ai/runpod) requires TCP+TLS for registration and QUIC
or TCP+TLS for the ongoing session — coexisting on one hub, not as
alternatives.

Rewrite:
- Multi-transport stated as decided: 'A hub MUST support TCP+TLS and
  QUIC endpoints simultaneously.' TCP+TLS accept loop (ADR-010 Am. 1)
  lives in the hub, shares the HandlerRegistry with the quinn endpoint.
- Channels substrate: the hub uses ChannelsAdapter/ChannelClient
  (from_connection primary, connect_quic convenience — ADR-080),
  not CallClient/CallAdapter directly. One channels connection per
  peer; CallAdapter runs on channel 0 (ADR-072).
- Transport-agnostic dial/accept: dial_worker_connection takes a
  Connection (one-way door); connect_quic_worker is a two-way-door
  convenience. supervise_worker takes a dial closure, not a SocketAddr.
- Identity over transports: fingerprint path (QUIC+raw-key,
  X.509 client cert) via resolve_from_fingerprint; bearer-token path
  (TCP+TLS no client cert, WebTransport, WebSocket) via
  resolve_from_token on the call first frame. Both resolve to the
  same PeerEntry (ADR-030/034).
- Worker registration flow (6 steps): provision → token → download →
  key gen → HTTP POST over TCP+TLS → channels connect. Step 4 is HTTP
  on HttpAdapter; step 6 is channels over QUIC or TCP+TLS. The hub
  creates a mixed-fingerprint PeerEntry (ADR-034 §3) at registration.
- OQ-58: worker registration flow — enrollment-token model,
  endpoint shape, register_worker API. Open (decision-ready, not
  blocked), one-way door, high priority.
- ADR-081: fix stale references to channels-hub/channels-worker
  sub-crates (the ADR's Decision says they don't exist; the References
  section said they did).

Review (architecture-reviewer): 3 critical (ADR-081 stale refs;
bearer-token extraction flow; assembly example QUIC-only +
unspecced into_connection), 7 warnings (registration 'or'→'both',
channel/control translation, OQ-52 interim, HubError variant naming,
RegistrationError definition, X.509-client-cert prose, adapter
ownership phrasing), 5 suggestions. All criticals and warnings #4/#7/#8
addressed; #5/#6/#9/#10 addressed; #11-15 noted as optional.

docs/architecture/README.md

@@ -171,7 +171,7 @@ adapter location map is now consistent: all HTTP-backed adapters
 | [crates/vault/encryption.md](crates/vault/encryption.md) | stable | AES-256-GCM, EncryptedData, key versioning, salt (Phase B reserved) |
 | [crates/vault/service.md](crates/vault/service.md) | stable | VaultServiceHandle lifecycle, direct dispatch, cache, error model |
 | [crates/vault/protocol.md](crates/vault/protocol.md) | stable | DerivedKey redaction, KeyType, serialization behavior |
-| [crates/hub/README.md](crates/hub/README.md) | draft | alknet-hub crate — aggregated peer env, connection lifecycle, worker supervision, service discovery |
+| [crates/hub/README.md](crates/hub/README.md) | draft | alknet-hub crate — multi-transport endpoint (TCP+TLS + QUIC), channels substrate (ADR-079 relay), worker registration flow (OQ-58), identity over transports, aggregated peer env, connection lifecycle, service discovery |
 | [crates/channels/README.md](crates/channels/README.md) | draft | alknet-channels crate — multiplexing proxy, 9-byte chunk format, N channels over one transport stream |
 | [crates/channels/overview.md](crates/channels/overview.md) | draft | Crate purpose, the multiplexing collapse, dependencies, transport agnosticism, WASM, relationship to existing crates |
 | [crates/channels/channels-wire.md](crates/channels/channels-wire.md) | draft | 9-byte chunk format, stream types, sentinels, framing disambiguation, wire-level invariants (REQ-CH-01..05) |