Complete M16 asset operator parity chain

docs/status/M16-GAP-00274.md

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+# M16-GAP-00274 Status
+
+status: done
+task: asset.catalogs_save operator LOCAL_EXACT slice
+updated: 2026-08-23 America/New_York
+
+scope:
+
+- The fixture links one object from external `WebGapCatalogsSave.blend`.
+  Blender desktop observes the no-active-asset-library boundary as `poll=false`,
+  records `CANCELLED` for `ASSET_OT_catalogs_save`, and confirms no Main
+  mutation. Save/reopen preserves the Library record exactly.
+- WASM/Main reads the same external Library with stable source path,
+  `EXTERNAL_REQUIRED` status, and an empty dependency closure. The comparator
+  confirms 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. Blender's expected missing
+  external-library warning is recorded by the comparator while the Library
+  state remains exact.
+
+nextTask: `M16-GAP-00275`