OVRTX color management

Keep exactly one display-transform owner:

• scene_linear_hdr: OVRTX supplies scene-linear data and the consumer applies Blender/OCIO view settings once.
• ldr_rgba8_display_passthrough: OVRTX supplies display-encoded pixels and the consumer must not apply Blender's display transform again.

Probe the allowed color_presentation_mode enum through ovrtx-current-scene-workflow, then set it with ovrtx-render-settings. Do not assume these identifiers exist in another add-on revision.

For the pinned official example, run the public ownership regression:

python3 scripts/run_ovrtx_color_presentation_probe.py \
  --output-dir /absolute/caller-owned/color-probe