Outer `signal.alarm` cap that fires even when trio's
`fail_after` is blocked by a shielded-await deadlock
(the bug-class-3 hang under MTF backends). Only armed
for fork-based spawners where the bug lives.
Deats,
- `_DIAG_CAP_S = fail_after_s + 5` — slightly larger than the
trio-native guard so it always loses when the in-band path works.
- `test_log.cancel()` breadcrumbs at each cancel-scope boundary so the
last-fired breadcrumb names the swallow point on hang.
- try/finally wrapping around each scope level for deterministic
breadcrumb emission.
- add `is_forking_spawner`, `set_fork_aware_capture` fixture params.
- rework `fail_after_s`: 4s for fork, 12s for trio (was 30/12).
Also,
- `test_sigint_both_stream_types`: `assert 0` -> `pytest.fail()`, add
TODO re `pytest.raises()`.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi
10db117864