Add opt-in `reap_subactors_per_test` fixture

Function-scoped, NON-autouse zombie-subactor reaper for
modules whose teardown is known-leaky enough to cascade-
fail every following test in a session.

Sibling to the autouse session-scoped `_reap_orphaned_subactors`. The
session-scoped one fires at session end — too late to save tests that
follow a hung/leaky test in the suite. The new fixture, opted into via
`pytestmark = pytest.mark.usefixtures(...)`, runs between tests in
a problem-module so a leftover subactor from test N can't squat on
registrar ports / UDS paths / shm segments needed by tests N+1,
N+2, ...

Intentionally NOT autouse — the fixture's presence on a module signals
"this module's teardown leaks; please root-cause instead of relying
forever on cleanup". A visibility-vs-convenience trade picked in favor
of the former.

Apply to `tests/test_infected_asyncio.py` since both recent full-suite
runs (parallel-tpt-proto + TCP-only) showed the cascade originating in
this file's KBI- and SIGINT-flavored tests under
`main_thread_forkserver`. Module-comment names the specific offenders so
future de-flake work has a starting point.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_infected_asyncio.py

@@ -32,6 +32,22 @@ from tractor.trionics import BroadcastReceiver
 from tractor._testing import expect_ctxc
 
 
+# Per-test zombie-subactor reaper. Opt-in (NOT autouse) —
+# see `tractor._testing.pytest.reap_subactors_per_test`'s
+# docstring for the full rationale. This module specifically
+# needs it because tests like
+# `test_echoserver_detailed_mechanics[KeyboardInterrupt]`
+# and the `test_sigint_closes_lifetime_stack[*]` matrix have
+# been observed to hang past pytest's wall-clock under
+# `main_thread_forkserver`, leaving subactor forks that
+# squat on registrar resources and cascade-fail every
+# subsequent test (`test_inter_peer_cancellation`,
+# `test_legacy_one_way_streaming`, etc.).
+pytestmark = pytest.mark.usefixtures(
+    'reap_subactors_per_test',
+)
+
+
 @pytest.fixture(
     scope='module',
 )

tractor/_testing/pytest.py

@@ -327,6 +327,55 @@ def _reap_orphaned_subactors():
         reap(pids, grace=3.0)
 
 
+@pytest.fixture
+def reap_subactors_per_test():
+    '''
+    Per-test (function-scoped) zombie-subactor reaper —
+    **opt-in**, NOT autouse.
+
+    When a test's teardown fails to fully cancel its actor
+    tree (e.g. an asyncio cancel-cascade times out under
+    `main_thread_forkserver`, pytest hits its 200s wall-
+    clock and abandons), the leftover subactor lingers as a
+    direct child of `pytest` and squats on whatever
+    registrar port / UDS path / shm segment it had bound.
+    Subsequent tests trying to allocate the same resource
+    fail — and with backends that bind a session-shared
+    `reg_addr`, that means EVERY following test in the
+    suite cascades. The session-scoped sibling
+    (`_reap_orphaned_subactors`) only kicks in at session
+    end which is too late to save the cascade.
+
+    Apply at module-level on the topically-problematic
+    test files via:
+
+    ```python
+    pytestmark = pytest.mark.usefixtures(
+        'reap_subactors_per_test',
+    )
+    ```
+
+    Or per-test via the same `usefixtures` mark on a
+    specific function. Intentionally NOT autouse so the
+    fixture's presence on a module signals "this module's
+    teardown is known-leaky enough to contaminate
+    siblings"; the visibility helps future-us track down
+    root causes rather than burying them under blanket
+    cleanup.
+
+    '''
+    import os
+    parent_pid: int = os.getpid()
+    yield
+    from tractor._testing._reap import (
+        find_descendants,
+        reap,
+    )
+    pids: list[int] = find_descendants(parent_pid)
+    if pids:
+        reap(pids, grace=3.0)
+
+
 @pytest.fixture(scope='session')
 def debug_mode(
     request: pytest.FixtureRequest,