Add semantic shape labels to inventory.py

Update `inventory.py` to assign semantic labels (title, subtitle, body, agenda_item_1, card_1_header, etc.) to shapes based on heuristics.

## Details
- Analyze shape position, size, and text content to infer semantic role
- Assign meaningful labels like `title`, `subtitle`, `body`, `agenda_item_1`, `card_1_header`, etc.
- Output enriched inventory JSON with semantic keys for each shape
- Labels should be deterministic and based on layout heuristics (position on slide, font size, placeholder type)

## Part of: PPTX Skill v6.0 Overhaul