Harden cancel-ack hard-kill escalation

Two defensive fixes around the `Portal.cancel_actor()` +
`_try_cancel_then_kill()` escalation from `34f333a0`
"Escalate cancel-ack timeouts to `proc.terminate()`" (the
`trionics.start_or_cancel` follow-up); surfaced by
`/code-review high` on #462,

- guard `proc.terminate()` for backends whose `proc` slot
  isn't a `Process` — the future `subint` backend stores an
  `int` interp-id, so escalation would `AttributeError`
  instead of hard-killing; now it logs + no-ops.
- swap `assert cs.cancelled_caught` for an
  `if cs.cancelled_caught and raise_on_timeout:` guard so an
  unexpected shielded-scope exit returns a soft `False`
  rather than crashing `cancel_actor()` mid-teardown.

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

tractor/runtime/_portal.py

@@ -334,8 +334,11 @@ class Portal:
             # `move_on_after` fired — peer didn't ack within
             # bounded window. Behaviour depends on
             # `raise_on_timeout`:
-            assert cs.cancelled_caught
+            if (
-            if raise_on_timeout:
+                cs.cancelled_caught
+                and
+                raise_on_timeout
+            ):
                 raise ActorTooSlowError(
                     f'Peer {peer_id} did not ack `Actor.cancel()`'
                     f'-RPC within bounded wait of '
@@ -344,6 +347,11 @@ class Portal:
 
             # legacy fire-and-forget path: log + return False so
             # the caller can decide whether to escalate.
+            #
+            # NOTE, we also land here in the (unexpected) case where
+            # the shielded `move_on_after` block exits WITHOUT
+            # `return True` and WITHOUT the deadline firing — prefer
+            # a soft `False` over an `assert`-crash mid-teardown.
             log.debug(
                 f'May have failed to cancel peer?\n'
                 f'\n'

tractor/runtime/_supervise.py

@@ -151,7 +151,20 @@ async def _try_cancel_then_kill(
             f'  reason: {too_slow}\n'
             f'-> escalating to `proc.terminate()` (hard-kill)\n'
         )
-        proc.terminate()
+        # XXX, the `subint` backend stores an `int` interp-id in the
+        # `proc` slot (not a `Process`), so it has no `.terminate()`.
+        # Guard here so a cancel-ack timeout doesn't `AttributeError`
+        # once that backend lands; its hard-kill path is a TODO.
+        if hasattr(proc, 'terminate'):
+            proc.terminate()
+        else:
+            log.error(
+                f'Cannot hard-kill sub-actor — backend proc-handle '
+                f'{proc!r} ({type(proc).__name__!r}) has no '
+                f'`.terminate()`!\n'
+                f'  uid: {subactor.aid.reprol()!r}\n'
+                f'TODO: per-backend cancel-escalation.\n'
+            )
 
 
 class ActorNursery: