Add Reconstruction metrics and DriveStudio plugin

[EricLiuhhh]

Summary

• Add Reconstruction metric support for photometric error, geometric discrepancy, novel-view fidelity, and novel-
  view consistency.
• Add a DriveStudio Reconstruction backend that trains, renders, and exports 4DGS assets for WorldLens evaluation.

Notes

• DriveStudio is vendored under worldbench/third_party/drivestudio.
• WorldLens owns metric computation; DriveStudio only trains, renders, and exports assets.
• Geometric discrepancy uses GT Grounded-SAM2 masks from processed_<gt-method>.
• The default DriveStudio training config is configs/omnire.yaml.