Generated feature_requirements and traceability_report pages reference existing images at paths that are not staged

## Summary

Images referenced from requirement text exist and are declared via `image_srcs`, but generated documentation pages still emit `image.not_readable` warnings because the generated output tree does not stage them at the paths used by the generated RST.

## Observed behavior

Two related cases were observed:

1. **feature_requirements pages**
Generated pages reference paths like:
`feature_requirements/img/.png`

but staged outputs land under deeper transitive paths or different locations.

2. **traceability_report pages**
Generated Lobster RST pages keep `.. image:: ...` directives from requirement text, but the corresponding images are not copied into the generated `traceability_report/...` tree at all.

## Expected behavior

If an image is referenced from requirement text and provided through `image_srcs`, generated pages should be able to resolve it without `image.not_readable` warnings.

## Why this looks like a tooling bug

- The source images exist in the repo.
- The warnings persist in generated pages, not because the source files are missing, but because generated staging paths do not match emitted image references.
- The mismatch appears after score_tooling doc generation / report generation.

## Impact

Migrated requirements with valid inline images produce noisy warnings and partially broken generated docs even when the images are present and wired.

## Notes

This issue is distinct from genuinely missing source images.
The bug here is specifically about **existing images not being staged where generated pages expect them**.

## Suspected areas

- artifact path computation for requirement doc staging
- Lobster RST multi-page report generation and asset copying