Ensure empty parent with child works show in IIIF viewer along with metadata

[kirkkwang]

Story

A parent work with no files of its own (only child works carry files) should render a working IIIF manifest so the viewer on the public show page displays the children's images. Child work metadata (the "page" level) should appear in the manifest alongside the parent's top-level metadata.

This is a presenter concern: no reindex expected, not migration-blocking. Applies to all seven work types.

Sample Data

compoundMultiWorkExample.csv

Acceptance Criteria

• A parent work with zero files and one or more child works produces a valid IIIF manifest
• The manifest includes file references from child works
• Child work metadata appears at the appropriate level in the manifest
• Parent metadata appears at the top level of the manifest
• The public show page viewer renders the child images from the manifest

Screenshots / Video

Testing Instructions and Sample Files

• Create a parent work (any type) with no attached files
• Attach one or more child works that have files
• Visit the parent's public show page and confirm the viewer loads and displays child images
• Inspect the IIIF manifest to verify both parent and child metadata are present at their respective levels

Notes

Current behavior untested; may already partially work. The viewer itself (UV) is a separate concern from manifest generation.