Continue Blender Web parity task handoff

docs/status/M16-GAP-00221.md

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+# M16-GAP-00221 Status
+
+status: done
+task: armature.collection_add operator LOCAL_EXACT slice
+updated: 2026-08-22 America/New_York
+
+scope:
+
+- The minimal fixture contains one armature bone assigned to `WebGapArmatureCollectionAddExisting`.
+- Desktop runs `ARMATURE_OT_collection_add` with `poll=true` and `FINISHED`, creating the `Bones` collection as index 1 and preserving the existing bone membership through save/reopen.
+- The existing Main reader exposes both bone collections and their member IDs; no new reader field, data-block, editor, or browser behavior was added.
+
+evidence:
+
+- The generator, desktop checker, npm comparator, direct comparator, and task-context checks passed. Repeated checker runs recognize the persisted collection and remain idempotent.
+- WASM/Main observes the same collection names, indices, and memberships, preserves them 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-00222`