Reorganize Blender Web execution around capabilities

docs/status/M16-GAP-00280.md

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+# M16-GAP-00280 Status
+
+status: done
+task: asset.mark_single operator LOCAL_EXACT slice
+updated: 2026-08-24 America/New_York
+
+scope:
+
+- The minimal fixture contains one marked object, `WebGapAssetMarkSingleObject`,
+  with stable author, description, and `operator-mark-single` tag metadata.
+  Blender desktop observes `asset.mark_single` as `poll=false`, records
+  `CANCELLED`, and confirms no Main mutation. Save/reopen preserves the
+  metadata exactly.
+- WASM/Main reads the same fixture through the existing Scene IR asset metadata
+  path. The comparator confirms the object metadata matches desktop, survives
+  save/reopen, and rejects malformed input without changing Main.
+
+evidence:
+
+- Fixture generation, desktop checker, npm generated-gap comparator, direct
+  comparator, and task-context handoff checks passed with exit code 0.
+  Chromium-only policy was preserved and no Firefox/WebKit run was made.
+- This task adds only the `mark_single` fixture/checker/comparator evidence;
+  the reader, Scene IR protocol, and WASM artifacts are the already verified
+  asset metadata path from M16-GAP-00279.
+
+nextTask: `M16-GAP-00281`