Scale `test_lifetime_stack` deadline for CI

`test_lifetime_stack_wipes_tmpfile` guards spawn+teardown with a
hard-coded `trio.move_on_after()` (1.6s / 1s) that isn't scaled
for slow CI. On a noisy macOS runner the `error_in_child=True`
case times out before the child error propagates, so the scope
cancels and `assert not cs.cancel_called` flips — reddening the
(required) macOS leg. Same unscaled-deadline class `main` already
fixed for `test_dynamic_pub_sub`.

- multiply the budget by `cpu_perf_headroom()` (`tests/conftest`),
  the established deadline-headroom helper (3x on macOS CI, a
  1.0 no-op locally / on un-throttled linux).

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

tests/test_runtime.py

@@ -73,6 +73,11 @@ async def test_lifetime_stack_wipes_tmpfile(
         1.6 if error_in_child
         else 1
     )
+    # scale for slow/noisy CI (esp. macOS) so the child error
+    # propagates before the deadline; otherwise `move_on_after`
+    # cancels first and flips the `error_in_child=True` assert.
+    from .conftest import cpu_perf_headroom
+    timeout *= cpu_perf_headroom()
     try:
         with trio.move_on_after(timeout) as cs:
             async with tractor.open_nursery(