Add preview renders and upload artifacts

[justinleeyang]

Summary

• Render top-down + angled preview images and an orbit preview video after export (best-effort; does not fail the job).
• Emit camera path JSONs and document preview outputs.
• Upload preview artifacts from the runner with dedicated data types.
• Add plyfile dependency for PLY bounds.

Testing

• python -m py_compile run.py
• cargo fmt --all
• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings

Notes: preview video rendering requires fmpeg and nerfstudio runtime; not exercised locally.

[justinleeyang]

Quick follow-up: this branch is ready for review. I also ran the local checks successfully:\n\n- python -m py_compile run.py\n- cargo fmt --all\n- cargo test --workspace --all-features\n- cargo clippy --workspace --all-targets --all-features -- -D warnings\n\nThe preview rendering remains best-effort, so the main splat pipeline still succeeds even if preview generation is unavailable in a given runtime. Happy to adjust naming or artifact types if you want a different convention.

[justinleeyang]

I rebased this branch on the latest develop and resolved the README conflict. The PR should be mergeable now. Let me know if you prefer the output artifacts list in a different doc location.