Mv core impl `tractor_diag.xsh` to `_testing.trace`

Extract all pure-Python diagnostic helpers (`dump_proc_tree`,
`dump_hung_state`, `scan_bindspace`, `dump_all`, `resolve_pids`,
`ensure_sudo_cached`, etc.) from the xonsh xontrib into a new
`tractor/_testing/trace.py` module so the same logic is callable
from both the `acli.*` terminal aliases AND in-test capture-on-hang
fixtures.

Deats,
- `_testing/trace.py`: new module (1171 lines) — proc-tree walker,
  hung-state dumper, bindspace scanner, `dump_all()` snapshot
  archiver, `AFKAlarmTimeout` exc, `fail_after_w_trace()` async CM
  (trio `fail_after` + auto-snapshot on `TooSlowError`),
  `afk_alarm_w_trace()` sync CM (`signal.alarm` + snapshot on
  `SIGALRM`), plus pytest fixture wrappers for both.
- `_testing/pytest.py`: re-export the two fixtures via `from .trace
  import` so pytest plugin-discovery picks them up.
- `tractor_diag.xsh`: thin terminal wrappers that import from
  `_testing.trace` — drops ~627 lines of inline impl. Add
  `acli.dump_all` alias for full snapshot-bundle CLI access.

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

(cherry picked from commit 7509e313ff)

(factored: the xontrib-side move-out hunk rides with the diag-xontrib segment)

tractor/_testing/pytest.py

@@ -38,6 +38,20 @@ import tractor
 from tractor.spawn._spawn import SpawnMethodKey
 import trio
 
+# Re-export `_testing.trace`'s pytest fixtures so they're
+# picked up by pytest's plugin-discovery (this module is
+# loaded via `pytest_plugins` from `pyproject.toml`). The
+# `noqa: F401` annotations make linters tolerate the
+# unused-looking imports — they're load-bearing for pytest
+# discovery. The fixtures share their `name=` kw with the
+# underlying CM functions; the python-level identifiers
+# below carry the `_fixture` suffix to avoid module-scope
+# collision (see `_testing/trace.py` for details).
+from .trace import (  # noqa: F401
+    afk_alarm_w_trace_fixture,
+    fail_after_w_trace_fixture,
+)
+
 # Spawn-backend keys which may appear in `skipon_spawn_backend`
 # marks ahead of the named backend actually being registered in
 # `tractor.spawn._spawn.SpawnMethodKey`; such marks are inert