Continue Blender Web parity task handoff

docs/status/M16-GAP-00255.md

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+# M16-GAP-00255 Status
+
+status: done
+task: armature.select_mirror operator LOCAL_EXACT slice
+updated: 2026-08-23 America/New_York
+
+scope:
+
+- The minimal fixture contains a selected `.L` bone, an unselected `.R` mirror,
+  and an unrelated center bone. Blender desktop runs
+  `ARMATURE_OT_select_mirror(only_active=false, extend=false)`, mirroring the
+  selection to `.R` and preserving the unrelated bone and geometry.
+- WASM/Main reads the same fixture. The comparator confirms desktop/WASM parity
+  and save/reopen stability.
+
+evidence:
+
+- Fixture generation, desktop checker, npm comparator, direct comparator,
+  malformed-input negative, save/reopen check, and task-context handoff passed.
+- Repeated checker execution is idempotent (`SKIPPED_ALREADY_APPLIED` after
+  the first run); Chromium-only policy was preserved and no Firefox/WebKit run
+  was made.
+
+nextTask: `M16-GAP-00256`