Support Mail Envelope Index schema

[pheidon]

Summary

• add a Mail metadata adapter for Apple Mail Envelope Index tables when synthetic metadata tables are absent
• keep mail account/mailbox/recent output aligned to the existing metadata JSON shape
• report non-SQLite Music library stores as unsupported schema instead of raw sqlite failure
• add regression coverage for Apple Mail Envelope Index shape and Music non-SQLite handling

Validation

• git diff --check
• swift test could not run on Pheidon: swift is not installed; GitHub CI is expected to validate on the self-hosted Swift runner

Privacy

• no live Mail or Music rows were read or printed; tests use synthetic fixture data only