v0.17.3

Added

• Model schemas now contain a deterministic vertex for every declared field,
  with the field's didactic sort, structural kind, requiredness, usage mode,
  axioms, serialized name, and nominal or structural identity. Scalar,
  reference, embedded, and tagged-union fields can thus be addressed
  independently by panproto VCS operations.
• Repository.blame_field() exposes first-parent field attribution as a typed
  Blame record. Attribution follows a field through committed schema
  migrations, including field renames, using panproto 0.75.0's migration-aware
  vertex blame.

Fixed

• Repository.add(Model) no longer erases field structure or
  FieldSpec.nominal. Same-named Models with different field shapes now commit
  different schemas, while Model data staging, replay, schema validation, and
  codec round trips continue to use the field-addressable representation.
  (#75)