refactor: modularize yaml2pptx into layered architecture

Refactor yaml2pptx.py from a 1,245-line monolithic script into a modular
architecture with clear separation of concerns. The entry point is now
127 lines, with business logic distributed across focused modules.

Architecture:
- core/: Domain models (elements, template, presentation)
- loaders/: YAML loading and validation
- renderers/: PPTX and HTML rendering
- preview/: Flask preview server
- utils.py: Shared utilities

Key improvements:
- Element abstraction layer using dataclass with validation
- Renderer logic built into generator classes
- Single-direction dependencies (no circular imports)
- Each module 150-300 lines for better readability
- Backward compatible CLI interface

Documentation:
- README.md: User-facing usage guide
- README_DEV.md: Developer documentation

OpenSpec:
- Archive refactor-yaml2pptx-modular change (63/70 tasks complete)
- Sync 5 delta specs to main specs (2 new + 3 updated)

.gitignore

@@ -263,5 +263,4 @@ pnpm-lock.yaml
 # !examples/
 # !assets/
 
-
 temp