Type the remaining niche examples

Finish the examples-typing sweep with the last non-docs-visible
scripts: `-> None` on the two `trio/` behavior-demo mains (plus a
`trio.TaskStatus` on `hold_lock_forever`) and nursery/portal typing
on `integration/mpi4py/inherit_parent_main.py`.

Leaves `concurrent_futures_primes` (a verbatim stdlib baseline) and
`integration/open_context_and_sleep` (its tractor nursery is
commented out) as-is, and the paren-group `trio.open_nursery()`
bindings unannotated (no clean spot for a preceding annotation).
Completes the examples-typing bullet in #472.

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

examples/integration/mpi4py/inherit_parent_main.py

@@ -33,15 +33,16 @@ async def main() -> None:
     rank = MPI.COMM_WORLD.Get_rank()
     print(f"[parent] rank={rank}  pid={os.getpid()}", flush=True)
 
+    an: tractor.ActorNursery
     async with tractor.open_nursery(start_method='trio') as an:
-        portal = await an.start_actor(
+        portal: tractor.Portal = await an.start_actor(
             'mpi-child',
             enable_modules=[child_fn.__module__],
             # Without this the child replays __main__, which
             # re-imports mpi4py and crashes on MPI_Init.
             inherit_parent_main=False,
         )
-        result = await portal.run(child_fn)
+        result: str = await portal.run(child_fn)
         print(f"[parent] got: {result}", flush=True)
         await portal.cancel_actor()
 

examples/trio/lockacquire_not_unmasked.py

@@ -32,8 +32,8 @@ async def acquire_singleton_lock(
 
 
 async def hold_lock_forever(
-    task_status=trio.TASK_STATUS_IGNORED
-):
+    task_status: trio.TaskStatus = trio.TASK_STATUS_IGNORED,
+) -> None:
     async with (
         tractor.trionics.maybe_raise_from_masking_exc(),
         acquire_singleton_lock() as lock,
@@ -46,7 +46,7 @@ async def main(
     ignore_special_cases: bool,
     loglevel: str = 'info',
     debug_mode: bool = True,
-):
+) -> None:
     async with (
         trio.open_nursery() as tn,
 

examples/trio/send_chan_aclose_masks_beg.py

@@ -134,7 +134,7 @@ async def main(
 
     raise_unmasked: bool = False,
     loglevel: str = 'info',
-):
+) -> None:
     tractor.log.get_console_log(level=loglevel)
 
     # the `.aclose()` being checkpoints on these