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/076-backpressure-channel-limits-id-reuse.md

@@ -2,7 +2,26 @@
 
 ## Status
 
-Accepted
+Accepted (amended 2026-07-18 by ADR-093 — backpressure is per-`channel_id`,
+not per-`(channel_id, stream_type)`; the channels layer has one reassembly
+buffer per channel, yielding a `BiStream` — see "Amendment (ADR-093,
+2026-07-18)" below)
+
+## Amendment (ADR-093, 2026-07-18)
+
+The bounded-buffer backpressure is per-`channel_id` (not per
+`(channel_id, stream_type)`). The channels layer has one reassembly
+buffer per channel (yielding a `BiStream`), not one per
+`(channel_id, stream_type)`. The 1 MiB default and the 256-channel cap are
+unchanged; the per-channel memory ceiling is 1 MiB (was up to 5 MiB for a
+TTY channel with 5 active stream_types under the per-stream_type model).
+This is a net improvement (lower memory ceiling per channel), not a
+regression. The bounded-buffer *approach* is unchanged; only the
+buffer granularity changes (per-channel, not per-stream_type).
+
+The body below describes the **original** (per-stream_type) shape; the
+amendment above is the operative decision. See ADR-093 for the resolution
+rationale.
 
 ## Context
 
@@ -134,7 +153,10 @@ doesn't change the wire format, so even that reversal is feasible.
 
 ## References
 
-- ADR-071: channels wire format (the chunks the buffers hold)
+- ADR-071: channels wire format (the chunks the buffers hold, as amended
+  by ADR-093)
+- ADR-093: channels pure channel multiplexing (amends this ADR —
+  per-channel reassembly buffer, not per-`(channel_id, stream_type)`)
 - ADR-073: channel lifecycle operations (`channel:too_many_channels` error)
 - ADR-075: ChannelManager (`buffer_cap`, `max_channels`, `next_id` fields)
 - `docs/research/alknet-channels/poc-summary.md` §POC Target 1 (backpressure