Complete M16 asset operator parity chain

docs/status/M16-GAP-00265.md

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+# M16-GAP-00265 Status
+
+status: done
+task: asset.assets_download operator LOCAL_EXACT slice
+updated: 2026-08-23 America/New_York
+
+scope:
+
+- The fixture contains one fake-user external Image with source path
+  `//assets/WebGapAssetsDownload.png`. Blender desktop enables online access,
+  confirms `ASSET_OT_assets_download.poll()` is false, and records the real
+  `CANCELLED` result with no Main mutation. Save/reopen preserves the image
+  resource fields exactly.
+- WASM/Main reads the same fixture as an external, unpacked Image with stable
+  `assetStatus=EXTERNAL`, `assetId`, source path, and dimensions. 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.
+
+nextTask: `M16-GAP-00266`