Add TSML commands, local validator, and SQL PK/FK metadata mapping

[clemensv]

Summary

• add �2tsml and s2tsml converters
• add local �alidate-tmsl command aligned with Microsoft TMSL docs (structural checks)
• capture SQL foreign keys in sql2a output and preserve PK/FK metadata through �2s/s2a
• emit TMSL isKey and
  elationships from Avro metadata
• add TSML tests and reference fixtures under est/tsml

Notes

• includes pending TSML branch changes as requested
• runtime pytest execution remains environment-dependent