ci: mypy ratchet -- keep type-clean modules clean (71 modules, 536 -> 442 errors) (#661)

* ci: mypy ratchet -- keep type-clean modules clean

mypy-clean.txt lists the 71 modules under src/ that type-check clean;
scripts/check_types.py runs mypy (--follow-imports=silent) on exactly
those files and fails on any error or a missing/unsorted/duplicate entry.
A new "Type check (mypy ratchet)" CI job runs it with mypy 1.20.2 and
pinned stubs; the manual pre-commit mypy hook now runs the same script
(a local hook, so mypy sees the installed requirements like CI does).

35 modules were made clean with annotation-only fixes: hints, typing.cast,
TYPE_CHECKING imports, implicit-Optional defaults made explicit, and
annotations widened (never guards removed) where mypy called a defensive
isinstance check unreachable. No runtime behaviour change.

mypy.ini: numpy and orjson are treated as Any (follow_imports=skip, also
for stubs). numpy 2.3+ stubs use 3.12 `type` statements that mypy won't
parse at python_version 3.10, and orjson is optional, so seeing its stubs
made the result depend on whether it was installed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* chore: annotate check_types.py's list-form mypy subprocess

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/common/render_gate.py

@@ -46,7 +46,7 @@ import sys
 import threading
 import time
 from collections import deque
-from typing import Any, Callable, Deque, List, Optional
+from typing import Any, Callable, Deque, List, Optional, cast
 
 from src.common.frame_timing import binding_releases_gil
 
@@ -103,8 +103,8 @@ def _held(lock: Any) -> bool:
     """Is ``lock`` held? RLocks report this thread's ownership; plain locks, anyone's."""
     is_owned = getattr(lock, "_is_owned", None)
     if is_owned is not None:
-        return is_owned()
-    return lock.locked()
+        return cast(bool, is_owned())
+    return cast(bool, lock.locked())
 
 
 class RenderGate: