docs(arch): add alknet-channels specs — ADRs 071-080, 7 spec docs, OQ-56/57

Phase 1 architecture for alknet-channels (multiplexing proxy on
alknet/channels). Grounded in the completed de-risk POC (28 tests) and the
landed ADR-070 (BidiStreamSource trait + Connection::from_source).

ADRs:
- 071: 9-byte chunk wire format (generalizes TTY's 5-byte)
- 072: channel 0 pre-negotiated as alknet/call (no special control plane)
- 073: channel lifecycle operations on the call protocol — channel/open,
  close, control, resources/subscribe; direction field pinned; subscribe
  from day one (not poll-for-v1 — StreamingHandler machinery exists)
- 074: ChannelBidiStreamSource implements BidiStreamSource (ADR-070);
  into_sub_streams() typed accessor for TTY; accept_bi() generic path
- 075: ChannelsAdapter + ChannelManager; REQ-CH-01..04 wire invariants
- 076: bounded-buffer backpressure (1 MiB), 256-channel cap, monotonic IDs
- 077: TTY inside channels uses sub-streams, not own wire format;
  amends ADR-052 scope to direct-connect TTY; channels feature on tty
- 078: two-pump shutdown-on-completion contract (handler-level)
- 079: hub relay translates channel 0, byte-forwards data channels
- 080: ChannelClient (QUIC-only); AlknetClient core extraction deferred (OQ-55)

Spec docs: overview, channels-wire, channels-connection, channels-adapter,
channel-operations, channel-client.

OQ-56 (full windowing) and OQ-57 (two-pump helper extraction) are genuine
deferred(scope) deferrals with concrete blocking conditions; the contracts
are decided, only the extensions are deferred.

Hedging audit converted three research hedges into decisions:
resources/subscribe (not poll), server-assigned IDs (not if-zero-RTT),
bounded-buffer (not if-HOL-becomes-a-problem).

docs/architecture/questions/056-full-channel-level-flow-control-windowing.md

@@ -0,0 +1,34 @@
+# OQ-56: Full Channel-Level Flow-Control Windowing
+
+- **Origin**: `docs/research/alknet-channels/phase-0-findings.md` §DP-5,
+  §OQ-CH-03; `docs/architecture/decisions/076-backpressure-channel-limits-id-reuse.md`
+- **Status**: deferred(scope)
+- **Door type**: two-way (additive — per-channel window tracking does not
+  change the wire format)
+- **Priority**: low
+- **Blocked on**: a real deployment observes head-of-line blocking on a
+  saturated channel where the bounded-buffer's stop-reading mitigation is
+  insufficient. The trigger is specific: a channel whose consumer is
+  persistently slower than its producer, causing the demux to stall that
+  channel's reads frequently enough that other channels' throughput is
+  measurably affected. The intended use cases (TTY, SSH, tunnels) are not
+  high-throughput in the HOL-blocking sense; the trigger requires a
+  high-throughput use case (e.g., file transfer over a tunnel) that
+  saturates a channel.
+- **Resolution**: Not yet decidable. The bounded-buffer backpressure
+  (ADR-076, default 1 MiB per `(channel_id, stream_type)`) is the decided
+  v1 mechanism — validated by the POC's 1 MiB `tunnel_large_payload` test
+  with no deadlock and no cross-channel blocking. Full channel-level
+  windowing (SSH-style sliding-window per channel) is an additive extension
+  that does not change the wire format; it adds per-channel window tracking
+  to the demux/mux. The decision to add it depends on whether the
+  bounded-buffer mitigation is sufficient in practice, which can only be
+  determined by a deployment that hits the limitation.
+- **What does NOT block on this**: the bounded-buffer mechanism is decided
+  and is the v1 implementation. Full windowing is an extension, not a
+  prerequisite. The channels crate ships with bounded-buffer backpressure;
+  full windowing is added if and only if the trigger condition is observed.
+- **Cross-references**: ADR-076 (bounded-buffer decision), ADR-071 (wire
+  format — unchanged by windowing extension),
+  `docs/research/alknet-channels/poc-summary.md` §POC Target 1
+  (backpressure validation)

docs/architecture/questions/057-two-pump-helper-extraction.md

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+# OQ-57: Two-Pump Helper Extraction to alknet-core
+
+- **Origin**: `docs/research/alknet-channels/poc-summary.md` §Issues
+  Surfaced #7; `docs/architecture/decisions/078-two-pump-shutdown-on-completion.md`
+- **Status**: deferred(scope)
+- **Door type**: two-way (additive — a helper function does not change any
+  API surface; handlers that inline the pattern continue to work)
+- **Priority**: low
+- **Blocked on**: a second two-pump handler existing, so the shape
+  convergence is observable. The tunnel handler is the first two-pump
+  consumer; the SSH `direct-tcpip` channel will be the second. Extracting
+  the helper from one consumer (the tunnel) would bake in a shape that the
+  second consumer (SSH) might not fit — the `fn pump_bidi<R, W>(recv: R,
+  send: W, ...) -> impl Future` signature is a cross-crate API surface if
+  it lives in `alknet-core`. The trigger is: two real two-pump handlers
+  exist and their inline implementations have converged on the same shape.
+- **Resolution**: Not yet decidable. The shutdown-on-completion *contract*
+  is decided (ADR-078) — a two-pump handler MUST shut down the opposite
+  sink when one pump completes, or it deadlocks. The *helper* extraction is
+  an implementation convenience: ~10 lines of inline code per handler vs. a
+  shared function in `alknet-core`. The contract is pinned; only the
+  extraction is deferred. The helper is extracted when two real consumers
+  exist and their shapes converge, so the extraction is grounded in two
+  implementations rather than guessed from one.
+- **What does NOT block on this**: the two-pump pattern is documented
+  (ADR-078) and the tunnel handler implements it inline. The SSH crate's
+  `direct-tcpip` handler will implement it inline too. Both work without a
+  shared helper. The friction is copy-paste with documentation (~10 lines),
+  not a missing capability.
+- **Cross-references**: ADR-078 (the shutdown-on-completion contract),
+  ADR-074 (the `accept_bi` that yields the stream pair the pumps operate
+  on), `docs/research/alknet-channels/poc-summary.md` §Issues Surfaced #7