mypy reports nothing, so the CI step blocks merges now

[alexarje]

Closes #350.

mypy musicalgestures/ is clean across all 69 source files, from 248 when that issue was opened, and the || true that made the step advisory is gone. No allowlist was needed in the end.

The last eight

Of a piece with the rest of the backlog:

• get_acceleration declared fps: int — the same mistake frame2ms carried. A frame rate is not an integer; 29.97 and 23.976 are ordinary.
• Two GPU objects created under a flag and used under the same flag, which mypy will not correlate.
• Three output names resolved in one branch and read in another.
• _pose_mediapipe and _rerender_pose_from_cache had no return annotation at all, so everything returning their result returned Any.

Why this is safe to gate on

The pin added in 3504a15 is the precondition. The count mypy reports moves with the checker as well as with the code: on identical source, 1.20.1 reports 169 errors where 2.3.1 reports 90. Blocking on an unpinned checker would let a release turn CI red with no change to the code at all.

Verification

682 tests pass, 4 skipped, on the same tree as the clean type check — rerun after the motion_mp merge rather than carried over from before it.

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