Rewrite build.py for template-first shape filling

Rewrite `build.py` to fill existing template shapes instead of creating new textboxes, with support for semantic keys.

## Details
- Instead of creating new shapes/textboxes, locate existing shapes in the template by semantic key
- Fill shapes with provided content while preserving template formatting and positioning
- Support semantic keys from the enriched inventory (title, subtitle, body, card_1_header, etc.)
- Preserve all existing shape properties (font, color, alignment, animations) when filling content
- Handle missing/extra keys gracefully

## Part of: PPTX Skill v6.0 Overhaul