docs: Phase 1 architecture spec — ADRs 001..006, spec docs, OQ promotion

The Phase 0 OQ ledger (OQ-TN-01..10) promoted into
docs/architecture/open-questions.md (now OQ-TN-01..14, with four
new Phase 1 residues). Six ADRs and four spec docs, all Draft, all
decision-shaped per the SDD process (ADRs carry the WHY; specs carry
the WHAT and reference by number):

ADRs:
- 001 open-params-layout: params = {resource, substrate} — the
  producer's stable name, not an address; wire-stable from the first
  consumer; unknown substrates fail loudly at schema time
- 002 alpn-strategy: single alk/tunnel ALPN (option A); substrate in
  params selects the framing; prior art (SSH/SOCKS5/udpgw) gives no
  reason for the split
- 003 codec-and-udp-framing: raw pass-through (stream) / mandatory
  [len: u16 BE] (UDP) — F-2 mandate recorded (the codec is mandatory
  for correctness, not cosmetics); len=0 = legal empty datagram;
  OQ-TN-13 resolved fail-loud (truncation)
- 004 no-backend-trait: halves functions at the assembly layer;
  listen is an establisher shape; hub re-produce deferred(OQ-TN-12);
  trait re-evaluated at the alknet ADR-078 convergence threshold
- 005 consumer-session-owns-teardown: TunnelSession with
  open/adopt/stream_halves/take_halves/pump_against/close/join/Drop —
  the W3 adopter gap closes structurally; Drop is the best-effort
  fallback
- 006 access-control-posture: the open gate is the boundary;
  TUNNEL_OPEN_SCOPE = tunnel:open (stable once published); op-level
  ACL; ownership seam; no allowlists in v1; identity = 0.7.0's
  precedence chain

Spec docs:
- overview.md: purpose, resource model, deps (alkcall 0.7.0,
  wasm-clean default, local feature), module map
- wire.md: the open op (params/reply/typed errors), the data plane
  by substrate, sentinels + half-close, byte diagrams
- producer.md: spec, establisher (dial + listen shapes), pump
  handler (pump_bidi inline, R-02), registration API, ACL posture
- consumer.md: TunnelSession (forward open + reverse adopt
  construction), data plane, teardown API (close/join/Drop incl.
  pump-less join semantics)
- bast.md: the BAST doc for the UDP codec (convention 12's trigger
  fired — the framing IS binary beyond pass-through)
- open-questions.md: OQ-TN-01..14; 01..10 promoted (faithful to the
  phase-0 ledger's final states), 11 partially resolved (collision
  domain = per-producer registry per ADR-001; lifecycle open),
  12 deferred(scope), 13 resolved fail-loud (ADR-003), 14 open

Verified by an architecture-reviewer pass (2 criticals, 6 majors,
10 minors — all fixed: ADR-001/003-vs-OQ decision-state contradictions
resolved; the Layer-2 mislabel corrected to the ADR-047 §4 per-session
fork; alktty/alknet ADR misattributions fixed; codec placement pinned
to the establisher (pump stays substrate-agnostic); reverse-path
construction named (TunnelSession::adopt + pump_against); security
posture promoted to ADR-006; BAST doc written; F-2 rationale deduped;
README tables completed; channel_id > 0; MTU wording fixed; ADR-047 §5
attributions corrected; impacts lines added to unresolved OQs; review/
ledger pointer paths added to README references).

Verification: doc set internal cross-refs all resolve; cargo test,
fmt --check, doc --no-deps clean

docs/architecture/decisions/004-no-backend-trait.md

@@ -0,0 +1,115 @@
+# ADR-004: No TunnelBackend Trait — Halves Functions at the Assembly Layer
+
+## Status
+
+Accepted (2026-09-07; resolves the OQ-TN-05 executable input + the hub
+re-produce thread)
+
+## Context
+
+alktty has a `TtyBackend` trait because backends (local PTY, docker,
+SSH) produce complex handles — `TtyHandle` carries
+stdin/stdout/stderr/exit-code quadruple + control — and the adapter
+pumps them. The Phase 0 question (OQ-TN-05): does alktunnels need a
+`TunnelBackend` trait, or is the two-pump handler + feature-gated
+substrate modules the whole story?
+
+The tunnel producer's substrate action is much narrower than a TTY
+backend's:
+
+- **Dial flows:** dial a target, get an `AsyncRead + AsyncWrite`
+  stream (or a datagram endpoint). A tunnel "handle" is just boxed
+  halves.
+- **Listen flows:** accept on a listener; per accept, the accepted
+  connection is the same boxed-halves shape.
+
+The UDP POC gave the executable answer for dial flows (2026-09-06):
+`pump_halves` is generic over boxed `(AsyncRead, AsyncWrite)` halves —
+TCP contributes `into_split()` halves, UDP contributes the `UdpHalf`
+socket adapter, the pump never knows which. "Produce boxed halves for
+a resource" is a function, not a trait.
+
+The one remaining thread was the hub re-produce composition (does a
+hub proxy that re-produces a consumed resource need a trait or a
+composition helper?). The reverse POC (2026-09-07) resolved the
+mechanics: a hub proxy is a consumer + producer composed at the
+assembly layer — the reverse POC's hub is exactly this shape (it
+consumes a channels connection and produces reverse-tunnel opens
+toward the worker); re-exporting (registering an openable for a
+resource the hub itself consumes) would add a
+`register_openable_with_establisher` whose establisher opens a
+consumer channel. Both halves are public-surface operations; no new
+mechanism is visible yet. Deferring the helper until a concrete
+re-produce consumer exists is the alknet ADR-078-style "genuine
+deferral" pattern
+pattern (OQ-TN-12).
+
+## Decision
+
+**No `TunnelBackend` trait in v1.** The inversion point is the
+assembly layer:
+
+- **The produce function.** The producer's establisher resolves
+  `(resource, substrate)` → boxed halves
+  (`Box<dyn AsyncRead + Send + Sync + Unpin>` ×2 — the `+ Sync` is
+  the `ChannelPlan` bound, reverse POC F-1) or a framed UDP adapter.
+  The dial itself is substrate code living in feature-gated backend
+  modules (`local` feature: real sockets), injected at assembly:
+  the establisher closure the assembly layer constructs closes over
+  the substrate dial function. The protocol crate never sees a
+  socket type.
+- **The listen variant** is an establisher shape, not a trait: a
+  producer that LISTENS (SSH `-R` far-side listener, or the hub's own
+  exposed port being produced to a third party) registers the same
+  open op; its establisher pops the next accepted connection from an
+  assembly-owned listener queue and returns it as the plan payload.
+  The listener lifecycle (bind, accept loop, cancel) is assembly
+  code — the protocol never binds (OQ-TN-04). The reverse POC's
+  worker (dial) and the forward POC's producer (dial) both rode this
+  shape; the listen establisher is the same plan-flow with a
+  different halves source.
+- **The pump handler is substrate-agnostic by construction:**
+  `pump_bidi(bidi, t_read, t_write)` over the downcast plan payload —
+  it cannot know whether the halves came from TCP, UDP, a unix
+  socket, or an in-process pipe, and must not.
+- **Datagram substrates** contribute a framed adapter (ADR-003)
+  instead of raw halves; the pump shape is unchanged.
+- **Re-evaluation trigger:** if a consumer crate demonstrates that
+  every assembly layer re-implements the same glue (registry + dial +
+  establisher construction + pump wiring) three or more times, extract
+  a `TunnelBackend`-shaped trait THEN (the alknet ADR-078
+  convergence-test threshold — extract when the shapes have converged
+  across consumers, not before)
+  — as an additive layer, not a v1 wire/ABI commitment. Until then
+  the function-plus-closure shape keeps the crate dependency-light
+  and wasm-clean.
+
+## Consequences
+
+- **The default crate stays wasm-clean** with no socket/platform
+  deps; `local` (sockets) and future backends (docker, process) are
+  feature-gated modules implementing dial/listen functions.
+- **Assembly layers write more glue than a trait would save** — the
+  accepted cost, validated twice (both POCs' producers are ~200 lines
+  including the UDP adapter).
+- **Plan payloads are `Send + Sync`** (reverse POC F-1, documented on
+  alkcall's `ChannelPlan` type in 0.7.0): socket-backed handles carry
+  `+ Sync` naturally; non-Sync handles (process pipes as boxed trait
+  objects) need a wrapper. The spec documents the constraint.
+- **The listen variant needs no new wire surface** — same open op,
+  same params, same typed errors (an empty listener queue during
+  establishment is `resource_shortage`; a closed listener is
+  `dial_failed`-class).
+
+## References
+
+- OQ-TN-05 (promoted), OQ-TN-12 (hub re-produce, deferred), OQ-TN-14
+  (unix/stdio placement)
+- Forward POC: `docs/research/poc-summary.md` §5 (halves-not-trait);
+  reverse POC: `docs/research/reverse-poc-summary.md` (the hub shape,
+  F-1)
+- alktty ADR-002 (`TtyBackend` — the contrast case: when a trait IS
+  warranted), alktty ADR-003 (backend placement — the feature-gate
+  pattern)
+- AGENTS.md conventions 4/7/14 (wasm-clean, substrate-agnostic,
+  feature flags)