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/open-questions.md

@@ -1,6 +1,6 @@
 ---
 status: draft
-last_updated: 2026-07-17
+last_updated: 2026-07-18
 ---
 
 # Open Questions
@@ -193,7 +193,8 @@ Door type is separate from whether a decision is made. A two-way door is a decis
 | OQ | Title | Status | Door | Pri |
 |----|-------|--------|------|-----|
 | [OQ-56](questions/056-full-channel-level-flow-control-windowing.md) | Full Channel-Level Flow-Control Windowing | deferred(scope) | two | low |
-| [OQ-57](questions/057-two-pump-helper-extraction.md) | Two-Pump Helper Extraction to alknet-core | deferred(scope) | two | low |
+| [OQ-57](questions/057-two-pump-helper-extraction.md) | Two-Pump Helper Extraction to alknet-Core | deferred(scope) | two | low |
+| [OQ-68](questions/068-channels-add-strip-api-shape.md) | Channels Add/Strip API Shape (Built-In vs. Utility) | open | two | low |
 
 ### alknet-tls