fix(test): remove two CI flakes — EPIPE on early-exit stdin, and an over-tight async watchdog [DEV-150, DEV-151]

[P4suta]

Two independent test-harness flakes that were turning main and PR runs red. Both are test bugs, not product bugs. Linear: DEV-150, DEV-151.

1. EPIPE when a CLI rejects stdin early (DEV-150)

Turned main red on 6d51600 (run 30050968242) — and reproduced on re-run.

zip_profile_rejects_standard_input_as_usage asserts that oxarchive package validate - refuses - as a usage error (exit 2). That behaviour is correct: the CLI exits immediately without reading stdin, closing the pipe. The run_stdin helper raced that exit — write_all(jar).expect("write stdin") returns BrokenPipe (EPIPE) whenever the child exits before the bytes land in the pipe buffer, so .expect panicked based purely on scheduling.

Fix: treat BrokenPipe as an expected outcome of feeding a process that may reject its arguments without consuming input; every other write error still panics. Stdin is scoped closed before wait_with_output, so children that do read stdin (deb_validates_from_standard_input) still see EOF. The test's assertions are unchanged.

2. Async deadlock watchdog too tight for emulated/loaded runners (DEV-151)

Failed big-endian (s390x, qemu) and test (macos-latest) on this PR's first run:

watchdog: `async_xz_never_deadlocks_under_slow_source` did not complete within 30s;
aborting the deadlocked test

Not a deadlock — the watchdog (added with DEV-124) mistook slowness for one. Driving the reader one byte per poll for 50 iterations honestly exceeds 30s on an emulated s390x and a loaded macOS runner, so the guard meant to kill flakes became one.

Fix: detection determinism comes from the park/unpark driver, not the repeat count — the executor has nothing else to poll, so a lost wakeup parks forever on the first iteration. Trim the loop to 8 repeats and raise the budget to 240s via a documented WATCHDOG_BUDGET. A real hang still aborts fast and attributably, well inside the job's timeout-minutes; honest slowness no longer trips it.

Verification

• package_cli → 9 passed; the previously flaky stdin test repeated 12/12 green.
• async_stream_v2 → 6 passed, suite now 0.30s natively (was dominated by the 50-iteration loop).

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