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/078-two-pump-shutdown-on-completion.md

@@ -128,8 +128,13 @@ existing, so shape convergence is observable).
 
 ## References
 
-- ADR-074: ChannelBidiStreamSource (the `accept_bi` that yields the stream
-  pair the pumps operate on)
+- ADR-074: ChannelBidiStreamSource (the `accept_bi` that yields the
+  `BiStream` the pumps operate on, as amended by ADR-093)
+- ADR-093: channels pure channel multiplexing (the `BiStream`-only
+  accessor decision this ADR's two-pump pattern builds on)
+- ADR-092: `BiStream` as the handler leaf (the transport-leaf decision —
+  `accept_bi` returns `BiStream`; the two-pump pattern calls
+  `tokio::io::split(bidi)` for its halves)
 - ADR-055: exit-chunk-is-last (the three-pump TTY invariant — the pattern
   this ADR does NOT touch)
 - `docs/research/alknet-channels/poc-summary.md` §Issues Surfaced #7 (the