Default `pytest` to use `--capture=sys`

Lands the capture-pipe workaround from the prior cluster of diagnosis
commits: switch pytest's `--capture` mode from the default `fd`
(redirects fd 1,2 to temp files, which fork children inherit and can
deadlock writing into) to `sys` (only `sys.stdout` / `sys.stderr` — fd
1,2 left alone).

Trade-off documented inline in `pyproject.toml`:
- LOST: per-test attribution of raw-fd output (C-ext writes,
  `os.write(2, ...)`, subproc stdout). Still goes to terminal / CI
  capture, just not per-test-scoped in the failure report.
- KEPT: `print()` + `logging` capture per-test (tractor's logger uses
  `sys.stderr`).
- KEPT: `pytest -s` debugging behavior.

This allows us to re-enable `test_nested_multierrors` without
skip-marking + clears the class of pytest-capture-induced hangs for any
future fork-based backend tests.

Deats,
- `pyproject.toml`: `'--capture=sys'` added to `addopts` w/ ~20 lines of
  rationale comment cross-ref'ing the post-mortem doc

- `test_cancellation`: drop `skipon_spawn_backend('subint_forkserver')`
  from `test_nested_ multierrors` — no longer needed.
  * file-level `pytestmark` covers any residual.

- `tests/spawn/test_subint_forkserver.py`: orphan-SIGINT test's xfail
  mark loosened from `strict=True` to `strict=False` + reason rewritten.
  * it passes in isolation but is session-env-pollution sensitive
    (leftover subactor PIDs competing for ports / inheriting harness
    FDs).
  * tolerate both outcomes until suite isolation improves.

- `test_shm`: extend the existing
  `skipon_spawn_backend('subint', ...)` to also skip
  `'subint_forkserver'`.
  * Different root cause from the cancel-cascade class:
    `multiprocessing.SharedMemory`'s `resource_tracker` + internals
    assume fresh- process state, don't survive fork-without-exec cleanly

- `tests/discovery/test_registrar.py`: bump timeout 3→7s on one test
  (unrelated to forkserver; just a flaky-under-load bump).

- `tractor.spawn._subint_forkserver`: inline comment-only future-work
  marker right before `_actor_child_main()` describing the planned
  conditional stdout/stderr-to-`/dev/null` redirect for cases where
  `--capture=sys` isn't enough (no code change — the redirect logic
  itself is deferred).

EXTRA NOTEs
-----------
The `--capture=sys` approach is the minimum- invasive fix: just a pytest
ini change, no runtime code change, works for all fork-based backends,
trade-offs well-understood (terminal-level capture still happens, just
not pytest's per-test attribution of raw-fd output).

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

(cherry picked from commit 4c133ab541)
(factored: dropped spawn-backend-only paths: tests/spawn/test_subint_forkserver.py + tractor/spawn/_subint_forkserver.py; the xfail-loosening bullet above no longer applies)

(factored: the test-file mark adjustments ride with the test-hardening segment)

pyproject.toml

@@ -217,6 +217,29 @@ addopts = [
   # don't show frickin captured logs AGAIN in the report..
   '--show-capture=no',
 
+  # sys-level capture. REQUIRED for fork-based spawn
+  # backends (e.g. `subint_forkserver`): default
+  # `--capture=fd` redirects fd 1,2 to temp files, and fork
+  # children inherit those fds — opaque deadlocks happen in
+  # the pytest-capture-machinery ↔ fork-child stdio
+  # interaction. `--capture=sys` only redirects Python-level
+  # `sys.stdout`/`sys.stderr`, leaving fd 1,2 alone.
+  #
+  # Trade-off (vs. `--capture=fd`):
+  # - LOST: per-test attribution of subactor *raw-fd* output
+  #   (C-ext writes, `os.write(2, ...)`, subproc stdout). Not
+  #   zero — those go to the terminal, captured by CI's
+  #   terminal-level capture, just not per-test-scoped in the
+  #   pytest failure report.
+  # - KEPT: Python-level `print()` + `logging` capture per-
+  #   test (tractor's logger uses `sys.stderr`, so tractor
+  #   log output IS still attributed per-test).
+  # - KEPT: user `pytest -s` for debugging (unaffected).
+  #
+  # Full post-mortem in
+  # `ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`.
+  '--capture=sys',
+
   # disable `xonsh` plugin
   # https://docs.pytest.org/en/stable/how-to/plugins.html#disabling-plugins-from-autoloading
   # https://docs.pytest.org/en/stable/how-to/plugins.html#deactivating-unregistering-a-plugin-by-name