snippets: export presets loop transform_apply per object; selected_objects override does not narrow the operator

[TMHSDigital]

Why it matters

All three engine export presets apply transforms with a per-object loop of bpy.ops.object.transform_apply under temp_override(selected_objects=[obj]). The override is meant to scope the operator to one object, but it doesn't: transform_apply acts on selected_editable_objects, which isn't overridden, so each iteration applies to the whole selection.

The final result happens to be correct only because applying is idempotent. The loop is O(N²) operator work, and it teaches a false pattern: that overriding selected_objects narrows an operator's target. It also breaks the repo's own rule rules/prefer-data-over-ops-in-loops.mdc, which none of the listed exceptions (:92-99) cover.

Evidence

snippets/export_preset_unreal.py:18-27 (same shape in export_preset_godot.py:17-26 and export_preset_unity.py):

for obj in list(bpy.context.selected_objects):
    ...
    with bpy.context.temp_override(
        object=obj, active_object=obj, selected_objects=[obj]
    ):
        bpy.ops.object.transform_apply(location=False, rotation=True, scale=True)

Reproduced headless on .scratch/5.2/blender-5.2.1-windows-x64/blender.exe (Blender 5.2.1 LTS): three selected cubes, each scale 2. The override above ran for cube a only:

PROBE1 after applying to only 'a': scales = [(1.0, 1.0, 1.0), (1.0, 1.0, 1.0), (1.0, 1.0, 1.0)]

All three had their scale applied, not just a.

Suggested approach

• Call transform_apply once for the whole selection, outside any loop. That is what the loop effectively does already.
• If a single-object scope is really needed somewhere, override selected_editable_objects=[obj]. Better, use the data API (obj.data.transform(matrix) plus resetting matrix_basis), which also satisfies the ops-in-loops rule.
• Make the same change in the Unity, Godot and Unreal presets, and in any skill text or template (templates/ai-asset-pipeline-template/) that copies this pattern.
• Consider adding a line to skills/operators or rules/prefer-temp-override-over-context-copy.mdc: overriding selected_objects does not scope operators that read selected_editable_objects.

Done when

• No preset calls bpy.ops inside a per-object loop.
• The override pattern that implies single-object scoping is gone or corrected everywhere it appears.