docs: resolve OQ-TN-14 — unix in via local, stdio out (alktty owns process stdio)

Split the last open substrate-placement question:

- Unix: ships with the local feature v1 (dial_unix — same halves
  shape as TCP; the wire enum already carried unix per ADR-001;
  the params task's schema list includes all three values)
- Stdio: OUT of scope — a spawned process's stdin/stdout/stderr IS
  alktty's pipe mode (LocalTtyBackend + tokio::process + Stdio::piped,
  alktty tty-local.md): three multiplexed logical streams + the
  exit-code control chunk (alktty ADR-004) + signal forwarding
  (REQ-TTY-02). A stdio bridge here would be alktty's runner mode
  with the terminal stripped out — a strictly worse duplicate that
  also drops the semantics that matter (a byte tunnel has neither
  exit codes nor signals). Remote command execution composes via
  alktty on the same channels substrate.

Updated: open-questions.md OQ-TN-14 (resolved), overview.md feature
gate + deps + OQ summary, producer.md OQ ref, OQ-TN-10 promotion
(#3 split), phase-0-findings + both POC summaries' resolution notes,
params task (schema enum includes unix), local-socket-halves task
(unix ships, stdio does NOT — with the composition rationale),
oq-tn-14-tracker task repurposed (boundary-maintenance: re-opens only
if a consumer needs stdio-without-process-semantics — which would
need its own ADR, or if the alktty/alktunnels boundary needs
sharpening).

Verified: taskgraph valid (12 tasks, no cycles)

docs/architecture/overview.md

@@ -101,7 +101,7 @@ alktunnels (default — wasm-clean)
 └── (no backend deps — sockets/process live in feature-gated modules)
 
 alktunnels (local feature) — non-wasm by design
-└── adds: tokio/net (TCP dial, UDP bind/connect, unix sockets — unix pending OQ-TN-14)
+└── adds: tokio/net (TCP dial, UDP connect, unix sockets — unix per OQ-TN-14)
 ```
 
 Same posture as alktty: the default crate is protocol-only and
@@ -115,7 +115,7 @@ trivial here, since the codec is a 2-byte length prefix).
 | Feature | Contents | Wasm |
 |---------|----------|------|
 | *(default)* | params, wire codec, open-op spec, establisher shapes, `TunnelSession` — protocol only | yes |
-| `local` | TCP/UDP/unix dial + listen helpers — halves-producing dial/listen functions (ADR-004; unix pending OQ-TN-14) | no |
+| `local` | TCP/UDP/unix dial + listen helpers — halves-producing dial/listen functions (ADR-004). Unix ships (OQ-TN-14); stdio does NOT — a spawned process's stdio is alktty's pipe mode (exit codes, signals), not a tunnel | no |
 
 ## Module Map
 
@@ -157,7 +157,8 @@ Key questions affecting this document:
 - **OQ-TN-11**: resource naming collision domain + lifecycle (partially resolved)
 - **OQ-TN-12**: hub re-produce composition helper (deferred(scope))
 - **OQ-TN-13**: UDP truncation semantics (resolved — fail-loud, ADR-003)
-- **OQ-TN-14**: unix/stdio substrate placement (open)
+- **OQ-TN-14**: unix/stdio placement (resolved — unix in via
+  `local`, stdio out: alktty's pipe mode owns process stdio)
 
 ## References