fix(cnn_v3): native OPEN_EXR_MULTILAYER + quiet render + flexible channel names

blender_export.py:
- Replace broken compositor FileOutput approach with native OPEN_EXR_MULTILAYER
  render output; all enabled passes included automatically, no socket wiring needed
- Suppress Fra:/Mem: render spam via os.dup2 fd redirect; per-frame progress
  printed to stderr via render_post handler

pack_blender_sample.py:
- get_pass_r: try .R/.X/.Y/.Z/'' suffixes + aliases param for Depth→Z fallback
- combined_rgba loaded once via ("Combined","Image") loop; shared by transp+target
- Remove unused sys import

HOW_TO_CNN.md: update channel table to native EXR naming (Depth.Z, IndexOB.X,
Shadow.X), fix example command, note Shadow defaults to 255 when absent

handoff(Gemini): blender pipeline now produces correct multilayer EXR with all
G-buffer passes; pack script handles native channel naming

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