Complete M16 asset operator parity chain

docs/status/M16-GAP-00266.md

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+# M16-GAP-00266 Status
+
+status: done
+task: asset.assign_action operator LOCAL_EXACT slice
+updated: 2026-08-23 America/New_York
+
+scope:
+
+- The fixture contains one fake-user `WebGapAssignAction` with a two-key
+  `location[0]` channel assigned to `WebGapAssignActionObject`. Blender
+  desktop observes the no-Asset-Shelf context as `poll=false`, records the
+  real `CANCELLED` result from `ASSET_OT_assign_action`, and confirms no Main
+  mutation. Save/reopen preserves the Action exactly.
+- WASM/Main reads the same fixture as one stable Action animation targeting
+  `object:WebGapAssignActionObject`. The comparator confirms both keyframes,
+  save/reopen stability, and malformed-input rejection without Main mutation.
+
+evidence:
+
+- Fixture generation, desktop checker, npm comparator, direct comparator, and
+  task context checks passed with exit code 0. Chromium-only policy was
+  preserved and no Firefox/WebKit run was made.
+
+nextTask: `M16-GAP-00267`