docs(arch): unweld ChannelClient from QUIC — from_connection primary, connect_quic convenience (ADR-080 amendment)

The channels protocol is transport-agnostic by design (ADR-071 substrate
modes; ADR-065 unwound the server-side QUIC-welding via
Connection::from_stream/from_bidi). ADR-080 had welded the client-side
one-way-door API to QUIC and masked it as a deferral ('QUIC-only
initially', 'can be generalized later') — anti-patterns #8/#9/#11
(door-type-as-deferral + resolved-with-escape-hatch). 'Can be
generalized later' meant 'can be rewritten later' — the expensive
reversal the one-way-door classification exists to prevent.

Fix: split the constructor surface.
- from_connection(connection: Connection) — transport-agnostic primary,
  one-way door. Mirrors server-side ChannelsAdapter::handle(Connection)
  and the existing CallClient::spawn_dispatch pattern.
- connect_quic(addr, credentials) — QUIC convenience, two-way door,
  additive. Future connect_tcp_tls / connect_webtransport join it
  without touching the one-way-door surface.

OQ-55 reframed: the deferred thing is the shared dial+TLS seam
(AlknetClient), not a QUIC-welded client API. The client take-over APIs
(CallClient::spawn_dispatch, ChannelClient::from_connection) are
transport-agnostic and decided; only the shared dial across transports
is blocked on a second transport's dial existing.

Files:
- decisions/080-channelclient.md: amendment section, Decision code
  block, Transport-agnostic by construction (replaces QUIC-only
  initially), Consequences, Door type, subscribe_resources added to
  Decision block (was referenced by Door type but missing)
- crates/channels/channel-client.md: from_connection primary API,
  Transport-agnostic by construction section, OQ-55 relationship
- crates/channels/overview.md, crates/channels/README.md,
  crates/core/README.md, README.md, open-questions.md,
  questions/055-...md: cross-reference summaries aligned

docs/architecture/crates/channels/overview.md

@@ -249,7 +249,7 @@ All design decisions are documented as ADRs in [decisions/](../../decisions/).
 | [077](../../decisions/077-tty-inside-channels.md) | TTY Inside Channels | Two modes (direct vs channels); 5 sub-streams; control bidirectional via 3/4 |
 | [078](../../decisions/078-two-pump-shutdown-on-completion.md) | Two-Pump Pattern | Shutdown-on-completion contract; handler-level |
 | [079](../../decisions/079-hub-relay-translate-not-forward.md) | Hub Relay | Translate channel 0, byte-forward data channels with ID rewrite |
-| [080](../../decisions/080-channelclient.md) | ChannelClient | Client side; QUIC-only; `AlknetClient` deferred (OQ-55) |
+| [080](../../decisions/080-channelclient.md) | ChannelClient | Client side; transport-agnostic `from_connection` primary, `connect_quic` convenience; `AlknetClient` dial-seam deferred (OQ-55) |
 | [081](../../decisions/081-channels-subcrate-decomposition.md) | Sub-Crate Decomposition | `channels-core` (pure multiplexer) / `channels-call` (call coupling + ChannelClient); hub and worker are consumers |
 
 ## Open Questions
@@ -257,9 +257,10 @@ All design decisions are documented as ADRs in [decisions/](../../decisions/).
 Open questions are tracked in [open-questions.md](../../open-questions.md).
 Key questions affecting this crate:
 
-- **OQ-55** (deferred(scope)): `AlknetClient` core extraction — blocked on
-  a second *transport's* client, not a second client. `ChannelClient` is
-  decided (ADR-080).
+- **OQ-55** (deferred(scope)): `AlknetClient` core **dial+TLS seam**
+  extraction — blocked on a second *transport's* dial. `ChannelClient`'s
+  API is transport-agnostic (`from_connection`); `AlknetClient` is the
+  shared *dial* across transports, not the channels protocol.
 - **OQ-56** (deferred(scope)): Full channel-level flow-control windowing —
   bounded-buffer is decided (ADR-076); full windowing is an extension
   blocked on a real HOL-blocking deployment observation.