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/decisions/081-channels-subcrate-decomposition.md

@@ -2,7 +2,25 @@
 
 ## Status
 
-Accepted
+Accepted (amended 2026-07-18 by ADR-093 — the 9-byte wire format is now
+8-byte; `ChannelSubStreams` / `SubStreamHandle` removed; the channels
+layer has no `stream_type` concept — see "Amendment (ADR-093, 2026-07-18)"
+below)
+
+## Amendment (ADR-093, 2026-07-18)
+
+The wire format in `channels-core` is now **8-byte** (not 9-byte); the
+`ChannelSubStreams` / `SubStreamHandle` typed destructure accessor is
+**removed** (the channels layer has no `stream_type` concept —
+`accept_bi` yields a `BiStream`, and the handler owns its sub-stream
+multiplexing). The channel 0 pre-negotiation in `channels-call` no
+longer constructs "reassembly buffers with `stream_types` [0, 1]" — it
+constructs one reassembly buffer for `channel_id = 0`, yielding a
+`BiStream` to the `CallAdapter`. The "What moves where" table's "9-byte
+wire format" row is now "8-byte wire format"; the `ChannelSubStreams`
+row is removed. The two-crate split (`channels-core` /
+`channels-call`), the dep graph, and the "hub and worker are consumers"
+principle are unchanged. See ADR-093 for the resolution rationale.
 
 ## Context
 
@@ -212,10 +230,15 @@ the dependency direction (hub/worker → channels, not channels → hub/worker).
 - ADR-003: crate decomposition (no-handler-depends-on-another-handler —
   preserved; the channels sub-crates depend on core/call, not on handlers)
 - ADR-071: channels wire format (revised — substrate simplification; the
-  wire format is in `channels-core`)
+  wire format is in `channels-core`; **amended by ADR-093 — 8-byte header,
+  no `stream_type`**)
+- ADR-093: channels pure channel multiplexing (amends this ADR — 8-byte
+  wire format; `ChannelSubStreams` / `SubStreamHandle` removed; the
+  channels layer has no `stream_type` concept)
 - ADR-072: channel 0 pre-negotiated (moves to `channels-call`)
 - ADR-073: channel lifecycle operations (move to `channels-call`)
-- ADR-074: ChannelBidiStreamSource (in `channels-core`)
+- ADR-074: ChannelBidiStreamSource (in `channels-core`; **amended by
+  ADR-093 — `into_sub_streams` removed, `accept_bi` yields `BiStream`**)
 - ADR-075: ChannelsAdapter and ChannelManager (split: core demux in
   `channels-core`, call coupling in `channels-call`)
 - ADR-079: hub relay (in `alknet-hub` — the hub crate consumes