Capture M16 gap execution artifacts

docs/status/M16-GAP-00156.md

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+# M16-GAP-00156 Status
+
+status: done
+task: anim.channels_collapse operator LOCAL_EXACT slice
+updated: 2026-08-21 America/New_York
+
+scope:
+
+- Action Editor `anim.channels_collapse` collapses all channels.
+
+evidence:
+
+- Blender desktop and WASM/Main preserve the same three location channels at frames 1, 3, and 5 after `all=true` collapse.
+- Save/reopen is exact; malformed Blend input is rejected without Main mutation.
+- Fixture generation, desktop checker, npm comparator, direct comparator, and artifact hashing passed.
+
+nextTask: `M16-GAP-00157`