ci: consolidate container image build matrices

[plexoos]

This PR consolidates the container build matrices so release and develop images are generated from the same configuration list, instead of maintaining separate duplicated matrix entries. It also refreshes the default CUDA/Geant4 image versions and keeps the README image table aligned with the workflow tags.

Changes

• Consolidates build-push.yaml around a shared target: [release, develop] and config matrix.
• Adds matrix-level default_alias metadata so alias tagging no longer depends on hard-coded version checks.
• Updates default container versions:
  • CUDA 13.0.2 -> 13.0.3
  • Geant4 11.4.1 -> 11.4.2
  • Adds Ubuntu 26.04 / CUDA 13.3.0 / OptiX 9.1.0 / CMake 4.3.4
• Updates PR build coverage to test both release and develop targets across the refreshed matrix.
• Keeps the Ubuntu 26.04 / CUDA 13.3.0 GPU test excluded until runner support is available.
• Adds workflow concurrency controls for push and release builds.
• Updates release tagging so latest and the short release tag are created as aliases for the default release configuration.
• Updates the Dockerfile defaults and makes uv use managed Python 3.13.
• Refreshes the README container image table to match the new image tags and aliases.

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