Test and validate PPTX skill end-to-end

Run inventory on template, create test content, build presentation, generate thumbnails.

## Details
- Run `inventory.py` on the template PPTX and verify semantic labels are correct
- Create test content using semantic keys for multiple slide layouts
- Run `build.py` to fill template slides with test content
- Generate thumbnails and visually verify output
- Verify formatting preservation (fonts, colors, alignment, animations)
- Test edge cases: missing keys, extra keys, empty content

## Part of: PPTX Skill v6.0 Overhaul