Add `SIGINT` cleanup to `spawn` fixture in `devx/conftest`

Convert `spawn` fixture to a generator and add post-test graceful
subproc cleanup via `SIGINT`/`SIGKILL` to avoid leaving stale `pexpect`
child procs around between test runs as well as any UDS-tpt socket files
under the system runtime-dir.

Deats,
- convert `return _spawn` -> `yield _spawn` to enable
  post-yield teardown logic.
- add a new `nonlocal spawned` ref so teardown logic can access the last
  spawned child from outside the delivered spawner fn-closure.
- add `SIGINT`-loop after yield with 5s timeout, then
  `SIGKILL` if proc still alive.
- add masked `breakpoint()` and TODO about UDS path cleanup

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

tests/devx/conftest.py

@@ -4,6 +4,7 @@
 '''
 from __future__ import annotations
 import time
+import signal
 from typing import (
     Callable,
     TYPE_CHECKING,
@@ -69,12 +70,15 @@ def spawn(
         import os
         os.environ['PYTHON_COLORS'] = '0'
 
+    spawned: PexpectSpawner|None = None
+
     def _spawn(
         cmd: str,
         **mkcmd_kwargs,
     ) -> pty_spawn.spawn:
+        nonlocal spawned
         unset_colors()
-        return testdir.spawn(
+        spawned = testdir.spawn(
             cmd=mk_cmd(
                 cmd,
                 **mkcmd_kwargs,
@@ -84,9 +88,32 @@ def spawn(
             # ^TODO? get `pytest` core to expose underlying
             # `pexpect.spawn()` stuff?
         )
+        return spawned
 
     # such that test-dep can pass input script name.
-    return _spawn  # the `PexpectSpawner`, type alias.
+    yield _spawn  # the `PexpectSpawner`, type alias.
+
+    if (
+        spawned
+        and
+        (ptyproc := spawned.ptyproc)
+    ):
+        start: float = time.time()
+        timeout: float = 5
+        while (
+            spawned
+            and
+            spawned.isalive()
+            and
+            (_time_took := (time.time() - start) < timeout)
+        ):
+            ptyproc.kill(signal.SIGINT)
+            time.sleep(0.01)
+        else:
+            ptyproc.kill(signal.SIGKILL)
+
+    # TODO? ensure we've cleaned up any UDS-paths?
+    # breakpoint()
 
 
 @pytest.fixture(