Continue Blender Web parity task handoff

docs/status/M16-GAP-00258.md

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+# M16-GAP-00258 Status
+
+status: done
+task: armature.separate operator LOCAL_EXACT slice
+updated: 2026-08-23 America/New_York
+
+scope:
+
+- The minimal fixture contains one selected bone and one unselected bone in a
+  single armature. Blender desktop runs `ARMATURE_OT_separate`, producing an
+  original armature with the retained bone and a separated armature with the
+  selected bone, stable through save/reopen.
+- WASM/Main reads both armature data-blocks from the same fixture. The
+  comparator confirms desktop/WASM partition parity, malformed-input rejection
+  without Main mutation, and save/reopen stability.
+
+evidence:
+
+- Fixture generation, desktop checker, npm comparator, direct comparator, and
+  repeated checker execution passed. The second checker run is idempotent
+  (`SKIPPED_ALREADY_APPLIED`); Chromium-only policy was preserved and no
+  Firefox/WebKit run was made.
+
+nextTask: `M16-GAP-00259`