Always pass `repl: PdbREPL` as first param to fixture

Oddly my env was borked bc this (apparently missed by `uv`?) sub-dep
wasn't installed and then `stackscope` was silently failing import and
caused the shield-pause test to also fail (since it couldn't match the
expected `log.devx()` on console). The import failure is not very
explanatory due to the `log.warning()`; change it to `.error()` level.

Also, explicitly import `_sync_pause_from_builtin` in
`examples/debugging/restore_builtin_breakpoint.py` to ensure the ref is
exported properly from `.devx.debug` (which it wasn't during dev of the
prior commit Bp).

tractor/devx/_stackscope.py

@@ -237,7 +237,7 @@ def enable_stack_on_sig(
     try:
         import stackscope
     except ImportError:
-        log.warning(
+        log.error(
             '`stackscope` not installed for use in debug mode!'
         )
         return None

tractor/devx/debug/_post_mortem.py

@@ -144,6 +144,7 @@ def _post_mortem(
     __tracebackhide__: bool = hide_tb
 
     with _maybe_open_repl_fixture(
+        repl=repl,
         repl_fixture=repl_fixture,
         boxed_maybe_exc=boxed_maybe_exc,
     ) as enter_repl:

tractor/devx/debug/_trace.py

@@ -90,6 +90,7 @@ if TYPE_CHECKING:
         Actor,
     )
     from ._post_mortem import BoxedMaybeException
+    from ._repl import PdbREPL
 
 log = get_logger(__package__)
 
@@ -109,6 +110,11 @@ _repl_fail_msg: str|None = (
 #     |_https://docs.python.org/3/library/contextlib.html#using-a-context-manager-as-a-function-decorator
 @cm
 def _maybe_open_repl_fixture(
+    repl: PdbREPL,
+    # ^XXX **always provided** by the low-level REPL-invoker,
+    # - _post_mortem()
+    # - _pause()
+
     repl_fixture: (
         AbstractContextManager[bool]
         |None
@@ -134,7 +140,10 @@ def _maybe_open_repl_fixture(
             repl_fixture
             or
             rt_repl_fixture
-        )(maybe_bxerr=boxed_maybe_exc)
+        )(
+            repl=repl,
+            maybe_bxerr=boxed_maybe_exc
+        )
 
     with _repl_fixture as enter_repl:
 
@@ -252,6 +261,7 @@ async def _pause(
         # nonlocal repl_fixture
 
         with _maybe_open_repl_fixture(
+            repl=repl,
             repl_fixture=repl_fixture,
         ) as enter_repl:
             if not enter_repl: