Capture M16 gap execution artifacts

docs/status/M16-GAP-00172.md

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+# M16-GAP-00172 Status
+
+status: done
+task: anim.clear_useless_actions operator LOCAL_EXACT slice
+updated: 2026-08-21 America/New_York
+
+scope:
+
+- `anim.clear_useless_actions` removes the fake-user-only Action with no F-Curves while preserving the used Action and the fake-user Action library with F-Curves.
+
+evidence:
+
+- Blender desktop and WASM/Main expose the same two preserved Actions, each with three location channels and frames 1, 3, and 5; the empty Action is absent.
+- Save/reopen preserves the animation snapshot exactly; malformed Blend input is rejected without Main mutation.
+- Fixture generation, desktop checker, npm comparator, direct comparator, and artifact hashing passed.
+
+nextTask: `M16-GAP-00173`