Continue Blender Web parity task handoff

docs/status/M16-GAP-00218.md

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+# M16-GAP-00218 Status
+
+status: done
+task: armature.bone_primitive_add operator LOCAL_EXACT slice
+updated: 2026-08-22 America/New_York
+
+scope:
+
+- The minimal fixture contains one empty armature object and a fixed 3D cursor at `(1.5, -2.0, 0.75)`.
+- Desktop runs `ARMATURE_OT_bone_primitive_add(name, length=2.5, align='UP', space='OBJECT', use_deform=false)` with `poll=true` and `FINISHED`, creating the named bone at the cursor and preserving its head/tail through save/reopen.
+- The existing Main reader exposes the same armature bone name and coordinates; 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 bone and remain idempotent.
+- WASM/Main observes the same cursor-created 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-00219`