Continue Blender Web parity task handoff

docs/status/M16-GAP-00216.md

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+# M16-GAP-00216 Status
+
+status: done
+task: armature.assign_to_collection operator LOCAL_EXACT slice
+updated: 2026-08-22 America/New_York
+
+scope:
+
+- The minimal fixture contains one armature with source and target bone collections, one parent bone, and one selected child initially assigned only to the source collection.
+- Desktop runs `ARMATURE_OT_assign_to_collection(collection_index=1)` with `poll=true` and `FINISHED`, assigning the child to the target collection and preserving both memberships through save/reopen.
+- The Main reader now exposes bounded armature `boneCollections` entries with collection name/index and member bone IDs. No other collection, editor, or browser behavior changed.
+
+evidence:
+
+- The reader rebuild, generator, desktop checker, npm comparator, direct comparator, and task-context checks passed. Repeated checker runs recognize the persisted assignment and remain idempotent.
+- WASM/Main observes source and target membership for the same child bone, preserves it across save/reopen, and rejects malformed Blend input without Main mutation.
+- Chromium-only policy was preserved; no Firefox/WebKit run was made.
+
+nextTask: `M16-GAP-00217`