test: readiness signals replace sleep-based timing (N4)

- signal tests use a marker-file readiness signal: the child's command
  is 'echo ready > <marker>; exec sleep 60', the test polls
  wait_for_file(marker, 5s) — marker exists = the shell exec'd, so the
  signal lands on the real target regardless of machine load. Applied
  in tests/pty.rs (both signal tests), tests/pipe.rs (SIGTERM), and
  the src/local unit tests; wait_for_file lives in tests/common.
- cancel-cleanup post-action sleeps became bounded polls for the
  child's death (kill(pid,0) -> ESRCH, 5s deadline) — faster and
  flake-proof in both directions.
- resize/cat-stdin tests need no readiness signal at all: the adapter's
  input pump processes chunks in order — the sleeps there were pure
  latency (integration suites now ~40ms, was 200-270ms).

docs/reviews/001-code-review.md

@@ -535,7 +535,7 @@ Highlights:
 | 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 | ✅ resolved (2026-09-05) |
-| N4 | sleep-based timing | readiness signals | small | low | open |
+| N4 | sleep-based timing | readiness signals | small | low | ✅ resolved (2026-09-05) |
 | N6 | MSRV unverified | CI MSRV job or bump | small | none | open |
 
 ### Resolution (2026-08-17, commit `9944153`)
@@ -652,14 +652,38 @@ warn + return, with no panic. `local/pty.rs` line coverage
 documented-unreachable arms plus `StdinSink`'s in-flight-parking path
 (a single write cannot fill the 64-slot channel).
 
+### Resolution (2026-09-05, N4 — readiness signals replace fixed sleeps)
+
+The signal tests now use a **marker-file readiness signal** (the
+pattern the cancel-cleanup tests already used): the child's command is
+`echo ready > <marker>; exec sleep 60`, and the test polls
+`wait_for_file(marker, 5s)` — the marker existing means the shell
+exec'd, so the signal lands on the real target regardless of machine
+load. Applied in `tests/pty.rs` (both signal tests), `tests/pipe.rs`
+(SIGTERM test), and the `src/local/` unit tests (`signal_int_kills_child`,
+`signal_reaches_process_group_child`, `unknown_signal_*`,
+`signal_term_kills_child`). `wait_for_file` lives in
+`tests/common/mod.rs`; `src/local/` copies are private test helpers
+(separate compilation units).
+
+The fixed **post-action** sleeps in the cancel-cleanup tests (wait
+after drop, then probe once) became **bounded polls** for the child's
+death (`kill(pid, 0)` → ESRCH) with a 5 s deadline — faster and
+flake-proof in both directions.
+
+The resize/cat-stdin tests needed no readiness signal at all: the
+adapter's input pump processes chunks in order and `resize` is safe
+whenever the control handle exists — the sleeps there were pure
+latency. Integration suites now finish in ~40 ms (was 200-270 ms
+each).
+
 ### Remaining (open)
 
-- **N4** — sleep-based timing in signal/cancel tests.
 - **N6** — MSRV unverified.
 
 ### Recommended Order (remaining)
 
-1. **N4 + N6** — test hardening and MSRV; defer until CI exists.
+1. **N6** — MSRV; defer until CI exists.
 
 ---