docs(adr): 093 — channels pure channel multiplexing (8-byte header, no stream_type)

Prune the channels spec to reflect the stream-unification resolution
(docs/research/stream-unification/findings.md): the channels wire format
goes from 9 bytes to 8 bytes, the channels layer no longer carries a
stream_type concept, into_sub_streams() is removed, and TTY always uses
its 5-byte format (carried transparently in the channels payload).

ADR-093 is the umbrella decision (the channels-layer consequence of
ADR-092's BiStream handler leaf): every channel is a BiStream, the
handler owns its sub-stream multiplexing, the channels layer routes by
channel_id only. Amends ADR-071 (8-byte header, no stream_type),
ADR-074 (into_sub_streams removed, accept_bi yields BiStream), reverses
ADR-077 (TTY always 5-byte), and the channels-facing clauses of
ADR-072/073/075/076/080/081. Adds ADR-092 forward-reference note
(into_sub_streams preservation subsequently reversed by ADR-093) and
the missing ADR-092 cross-reference on ADR-070.

Adds OQ-68 (add/strip API shape — built-in vs utility; the contract is
decided in ADR-093, the function surface is open; two-way door, low
priority, decision-ready when the channels crate's implementation
begins).

Rewrites the 7 channels spec docs (README, overview, channels-wire,
channels-connection, channels-adapter, channel-operations, channel-client)
to describe the post-amendment shape as current, with the 8-byte header,
the add/strip composition, single accept_bi accessor, BiStream per
channel, and TTY-always-5-byte.

Touch-up cross-references in hub README, client README, ADR-085, and
the OQ-45/47/65 question files (TTY-internal stream_type 3 →
STREAM_CTRL_IN; channels 9-byte → 8-byte).

docs/architecture/crates/hub/README.md

@@ -814,11 +814,16 @@ See [open-questions.md](../../open-questions.md) for full details.
   ALPN would serve the same role over QUIC/TCP without HTTP.
 - **OQ-65** (open): WebSocket carrying channels — whether the browser
   path extends from call-protocol-only (ADR-048) to full channels
-  (the 9-byte chunk format over WebSocket binary frames). If chosen,
+  (the 8-byte chunk format over WebSocket binary frames). If chosen,
   the browser is a first-class channels participant and the hub relay
   works unchanged for browser legs. The web endpoint advertises
   `alknet/channels` by default (ADR-086 §3 — the advertisement is
   settled; OQ-65 governs whether the browser path uses it).
+- **OQ-68** (open): Channels add/strip API shape — whether the 8-byte
+  header add/strip is built into the channels read/write path or
+  exposed as a standalone utility. The *contract* is decided (ADR-093);
+  the *function surface* is not. Does not block the hub (the hub uses
+  the `ChannelManager` interface either way).
 - **OQ-52** (open): `CallConnection::wait_for_close()` — the
   supervision loop needs a way to await connection close. The
   committed interim is polling `connection().accept_bi()` until