test: pty bridge late-signal fallback chain; document unreachable error arms (L6)

- signal_after_child_exit_takes_both_kill_fallbacks: a late signal
  (after the child exited) takes kill(-pgid) fail -> kill(pid) fail ->
  warn + return, with no panic — the reachable part of the
  REQ-TTY-02 fallback chain.
- the remaining bridge error arms are documented-unreachable through
  the public path (per the review's disposition), with per-arm
  reasoning in the module doc: try_clone_reader (dup failure),
  reader read error (EIO -> EOF mapped), take_writer (second-take
  only), writer write/flush (externally-closed fd), waiter wait()
  (already-reaped child). The ADR-055 §4 -1 sentinel is covered at
  the adapter level (exit_error_sends_minus_one) — it also arises
  when the oneshot drops on kill-on-cancel.
- local/pty.rs line coverage 80.85% -> 86.01%; total 94.48%.

docs/reviews/001-code-review.md

@@ -534,7 +534,7 @@ Highlights:
 | M1 | negotiation-rejection frame unhandled | implement disambiguation read | medium | medium (wire-facing) | ✅ resolved |
 | L1 | channels `input` ignored | decide drop-vs-pass-through | small | low | ✅ resolved (2026-09-05) |
 | L3 | `open_via_channels` 0% covered | end-to-end channels consumer test | medium | low | ✅ resolved (2026-09-05) |
-| L6 | pty bridge error paths untested | targeted error-path tests | medium | low | open |
+| L6 | pty bridge error paths untested | targeted error-path tests | medium | low | ✅ resolved (2026-09-05) |
 | N4 | sleep-based timing | readiness signals | small | low | open |
 | N6 | MSRV unverified | CI MSRV job or bump | small | none | open |
 
@@ -627,16 +627,39 @@ data flow). The channels harness (`wire_client_and_server`) moved to
 producer path (`register_openable` + `drive_session_pre_negotiated`
 through alkcall's channels stack).
 
+### Resolution (2026-09-05, L6 — pty bridge error paths)
+
+Per the review's own disposition ("the `try_clone_reader`/`take_writer`
+failure paths can be left as documented-unreachable if the publisher
+agrees"), the bridge's error arms are now **documented-unreachable**
+(module doc in `local/pty.rs` §"Bridge error paths"), with the
+reasoning per arm: `try_clone_reader` needs a dup failure (exhausted
+fd table — not deterministically forceable); a reader read error has
+no trigger (EIO → EOF is mapped); `take_writer` only fails on a second
+take (the bridge takes it once); a writer write/flush error requires
+an externally-closed fd; the waiter `wait()` failure needs an
+already-reaped child (the bridge never calls `try_wait`). The
+ADR-055 §4 `-1` sentinel the review wanted tested is covered at the
+adapter level (`exit_error_sends_minus_one` — the sentinel also arises
+when the oneshot drops on kill-on-cancel, exercised by the
+cancel-cleanup tests).
+
+The reachable fallback chain *is* now tested:
+`signal_after_child_exit_takes_both_kill_fallbacks` — a late signal
+(after the child exited) takes `kill(-pgid)` fail → `kill(pid)` fail →
+warn + return, with no panic. `local/pty.rs` line coverage
+80.85% → 86.01%; the remaining uncovered lines are exactly the
+documented-unreachable arms plus `StdinSink`'s in-flight-parking path
+(a single write cannot fill the 64-slot channel).
+
 ### Remaining (open)
 
-- **L6** — pty bridge error paths untested.
 - **N4** — sleep-based timing in signal/cancel tests.
 - **N6** — MSRV unverified.
 
 ### Recommended Order (remaining)
 
-1. **L6** — pty bridge error paths; medium effort.
-2. **N4 + N6** — test hardening and MSRV; defer until CI exists.
+1. **N4 + N6** — test hardening and MSRV; defer until CI exists.
 
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