Addresses review feedback on export_bundle:
- If subtitles_path or thumbnail_path is provided but the file is missing, the
tool now fails with an explicit error instead of silently producing a package
without that asset (which could ship an approved deliverable missing part of
its content).
- Default export location now stays inside the project workspace: when the
render lives at projects/<name>/renders/..., the bundle defaults to
projects/<name>/exports/ (alongside artifacts/, assets/, renders/) rather than
a repo-root exports/<name>/. export_dir remains an explicit override.
Tests cover both: missing optional asset errors, and the project-workspace
default path.
9bcccce2db