refactor: modular architecture with builtin lexicon support
- split lib.rs into parser, types, codegen, and builtin modules
- implement internal ref resolution (#localDef -> ClassName)
- implement external ref resolution (com.atproto.repo.strongRef -> ComAtprotoRepoStrongRef)
- bundle 81 com.atproto.* lexicons at compile time for automatic resolution
- add dynamic versioning from git tags in CI
- add tests for ref resolution
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Co-Authored-By: Claude <noreply@anthropic.com>