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/research/poc-summary.md

@@ -293,7 +293,9 @@ review and ADRs:
 2. **Unix-socket and stdio substrates** — the OQ-TN-10 #3 "cheap"
    item. The pump is substrate-agnostic by construction (finding #5);
    these are wiring, not mechanics. Fold into the real crate's test
-   suite rather than a separate POC.
+   suite rather than a separate POC. (Phase 1 resolution: unix in via
+   the `local` feature; stdio OUT — alktty's pipe mode owns process
+   stdio. OQ-TN-14.)
 3. **The reverse-flow (`-R`) advertisement** — OQ-TN-10 #2 remains
    open; the SSH `tcpip-forward` template in the survey is the
    lifecycle shape to validate there.