Continue Blender Web parity task handoff

docs/status/M16-GAP-00178.md

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+# M16-GAP-00178 Status
+
+status: done
+task: anim.keyframe_clear_button operator LOCAL_EXACT slice
+updated: 2026-08-21 America/New_York
+
+scope:
+
+- The minimal fixture animates one scalar custom RNA property, `clear_target`, on `WebGapAnimKeyframeClearButtonObject` at frames 1, 3, and 5. A foreground Properties context activates that RNA button and invokes `ANIM_OT_keyframe_clear_button(all=true)` (`poll=true`, `FINISHED`), leaving the Action with no F-Curves.
+
+evidence:
+
+- Desktop evidence records the property changing from three selected keyframes to none; WASM/Main reads the original channel as `["clear_target"][0]` and the cleared fixture with no matching animation. Save/reopen is exact and malformed Blend input is rejected without Main mutation.
+- Fixture generation, foreground desktop checker, npm comparator, direct comparator, and artifact hashing passed. The checker and comparator are idempotent for the already-cleared exact fixture while retaining the original `before` evidence.
+
+nextTask: `M16-GAP-00179`