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/072-channel-0-pre-negotiated-call.md

@@ -2,7 +2,27 @@
 
 ## Status
 
-Accepted
+Accepted (amended 2026-07-18 by ADR-093 — channel 0's `stream_types`
+field is removed; the channels layer has no `stream_type` concept; the
+call protocol's `EventEnvelope` framing is the channels payload, carried
+transparently — see "Amendment (ADR-093, 2026-07-18)" below)
+
+## Amendment (ADR-093, 2026-07-18)
+
+Channel 0's `stream_types` field (the `[0, 1]` active set) is **removed**.
+The channels layer has no `stream_type` concept (ADR-093) — it carries the
+call protocol's `EventEnvelope` framing (ADR-064) transparently in the
+8-byte header's payload. The call protocol's bidirectionality (client
+writes requests, server writes responses) is a call-protocol concern,
+not a channels-layer concern; the channels layer routes by `channel_id`
+only and yields a `BiStream` to the `CallAdapter`. The `CallAdapter`'s
+`accept_bi()` returns one `BiStream` (per ADR-092); the call protocol
+reads/writes `EventEnvelope` frames on it, exactly as on a top-level
+`alknet/call` connection.
+
+The body below describes the **original** (with `stream_types`) shape;
+the amendment above is the operative decision. See ADR-093 for the
+resolution rationale and the cross-ADR impacts.
 
 ## Context
 
@@ -121,7 +141,11 @@ unused; assigning them is additive).
 
 ## References
 
-- ADR-071: channels wire format (the 9-byte chunk header channel 0 uses)
+- ADR-071: channels wire format (the 8-byte chunk header channel 0 uses,
+  as amended by ADR-093)
+- ADR-093: channels pure channel multiplexing (amends this ADR —
+  channel 0's `stream_types` field removed; the call protocol's framing
+  is the channels payload, carried transparently)
 - ADR-073: channel lifecycle operations (registered on channel 0's
   `OperationRegistry`)
 - ADR-064: irpc never integrated — hand-rolled EventEnvelope framing (the