fix(blender_export): version detection + Blender 5.x warning + cleanup

- Use bpy.app.version for version detection instead of attribute sniffing
- Blender 5.0.x: warn that per-pass compositor routing is broken (Combined only);
  compositing_node_group path kept ready for when Blender fixes this upstream
- Remove all DEBUG prints and failed use_nodes=True experiment
- configure_scene() returns only discard_dir (compositor always configured)
- Move _SOCKET_ALIASES to module level; simplify slots/None fallback

handoff(Gemini): blender_export.py stable for Blender 4.5 LTS; Blender 5.x
path is forward-compatible but produces Combined-only output until upstream fix.

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