refactor: modularize secretspec-derive macro with compile-time validation

- Extract FieldInfo and ProfileVariant structs to encapsulate metadata
- Create modular architecture with profile_generation, secret_spec_generation, and builder_generation modules
- Add compile-time validation for Rust identifiers and keywords
- Replace HashMap with BTreeMap for consistent ordering
- Implement DRY principles throughout code generation
- Consolidate duplicate assignment and field generation logic
- Distinguish compile-time validation from existing runtime validation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: domenkozar