Arm `dump_on_hang` on `test_stale_entry_is_deleted`

Wrap the test's `trio.run(main)` in
`dump_on_hang(seconds=20)` so any future hang
regression captures a stack dump for triage instead
of wedging CI silently; under the default backends
it's a no-op safety net.

Includes a "KNOWN ISSUE" comment block documenting
the (future) `subint` backend hang classes observed
against this test during Phase B bringup (#379).

(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 4a3254583b)
(factored: kept only the tests/discovery/test_registrar.py part of
 "Doc `subint` backend hang classes + arm `dump_on_hang`"; dropped
 subint conc-anal docs + tests/test_subint_cancellation.py)

tests/discovery/test_registrar.py

@@ -14,6 +14,7 @@ import psutil
 import pytest
 import subprocess
 import tractor
+from tractor.devx import dump_on_hang
 from tractor.trionics import collapse_eg
 from tractor._testing import tractor_test
 from tractor.discovery._addr import wrap_address
@@ -562,4 +563,53 @@ def test_stale_entry_is_deleted(
                 await ptl.cancel_actor()
                 await an.cancel()
 
-    trio.run(main)
+    # TODO, remove once the `[subint]` variant no longer hangs.
+    #
+    # Status (as of Phase B hard-kill landing):
+    #
+    # - `[trio]`/`[mp_*]` variants: completes normally; `dump_on_hang`
+    #   is a no-op safety net here.
+    #
+    # - `[subint]` variant: hangs indefinitely AND is un-Ctrl-C-able.
+    #   `strace -p <pytest_pid>` while in the hang reveals a silently-
+    #   dropped SIGINT — the C signal handler tries to write the
+    #   signum byte to Python's signal-wakeup fd and gets `EAGAIN`,
+    #   meaning the pipe is full (nobody's draining it).
+    #
+    #   Root-cause chain: our hard-kill in `spawn._subint` abandoned
+    #   the driver OS-thread (which is `daemon=True`) after the soft-
+    #   kill timeout, but the *sub-interpreter* inside that thread is
+    #   still running `trio.run()` — `_interpreters.destroy()` can't
+    #   force-stop a running subint (raises `InterpreterError`), and
+    #   legacy-config subints share the main GIL. The abandoned subint
+    #   starves the parent's trio event loop from iterating often
+    #   enough to drain its wakeup pipe → SIGINT silently drops.
+    #
+    #   This is structurally a CPython-level limitation: there's no
+    #   public force-destroy primitive for a running subint. We
+    #   escape on the harness side via a SIGINT-loop in the `daemon`
+    #   fixture teardown (killing the bg registrar subproc closes its
+    #   end of the IPC, which eventually unblocks a recv in main trio,
+    #   which lets the loop drain the wakeup pipe). Long-term fix path:
+    #   msgspec PEP 684 support (jcrist/msgspec#563) → isolated-mode
+    #   subints with per-interp GIL.
+    #
+    #   Full analysis:
+    #   `ai/conc-anal/subint_sigint_starvation_issue.md`
+    #
+    #   See also the *sibling* hang class documented in
+    #   `ai/conc-anal/subint_cancel_delivery_hang_issue.md` — same
+    #   subint backend, different root cause (Ctrl-C-able hang, main
+    #   trio loop iterating fine; ours to fix, not CPython's).
+    #   Reproduced by `tests/test_subint_cancellation.py
+    #   ::test_subint_non_checkpointing_child`.
+    #
+    # Kept here (and not behind a `pytestmark.skip`) so we can still
+    # inspect the dump file if the hang ever returns after a refactor.
+    # `pytest`'s stderr capture eats `faulthandler` output otherwise,
+    # so we route `dump_on_hang` to a file.
+    with dump_on_hang(
+        seconds=20,
+        path=f'/tmp/test_stale_entry_is_deleted_{start_method}.dump',
+    ):
+        trio.run(main)