Add a `._trace.maybe_pause_bp()` for tpt-broken cases

Internal helper which falls back to sync `pdb` when the
child actor can't reach root to acquire the TTY lock.

Useful when debugging tpt layer failures (intentional or
otherwise) where a sub-actor can no longer IPC-contact the
root to coordinate REPL access; root uses `.pause()` as
normal while non-root falls back to `mk_pdb().set_trace()`.

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

tractor/devx/debug/_trace.py

@@ -1257,3 +1257,26 @@ async def breakpoint(
         api_frame=inspect.currentframe(),
         **kwargs,
     )
+
+
+async def maybe_pause_bp():
+    '''
+    Internal (ONLY for now) `breakpoint()`-er fn which only tries to
+    use the multi-actor `.pause()` API when the current actor is the
+    root.
+
+    ?! BUT WHY !?
+       -------
+
+    This is useful when debugging cases where the tpt layer breaks
+    (or is intentionally broken, say during resiliency testing) in
+    the case where a child can no longer contact the root process to
+    acquire the process-tree-singleton TTY lock.
+
+    '''
+    import tractor
+    actor = tractor.current_actor()
+    if actor.aid.name == 'root':
+        await tractor.pause(shield=True)
+    else:
+        tractor.devx.mk_pdb().set_trace()