Deeper parsing, stricter validation, more specialized model

For example:
- Validate function arguments
- Transform declarations inside `@page`, `@bottom-right`, etc. rules.
- ...

The issue provides no file names, tests, or defined acceptance criteria. Start by locating the ANTLR grammar, parser entry points, and model classes, then determine how function arguments and declarations in at-rules are currently handled. Done requires an agreed scope for deeper parsing, stricter validation, and the specialized model, backed by tests.

索引モデルが issue の本文から書いたものです。