Add JSON Type Definition (JTD, RFC 8927) support

[clemensv]

Feature request

Add conversion support for JSON Type Definition (JTD, RFC 8927).

Motivation

JTD is a simpler JSON schema language with growing adoption; direct support would complement existing JSON Schema support.

Suggested scope

• Import: jtd2a (JTD -> Avrotize Schema)
• Export: ^G2jtd (Avrotize Schema -> JTD)
• Map forms (properties, optionalProperties, ^[lements, �alues, discriminator, ^[num, ype,
  ef)
• Preserve unsupported semantics via metadata extensions

Acceptance criteria

• Commands + docs
• Fixtures for each JTD form
• Roundtrip behavior documented

Additional requirement: JSON Structure conversions

Please also include JSON Structure conversion support:

• jtd2s (JTD -> JSON Structure)
• s2jtd (JSON Structure -> JTD)
• Ensure coverage for all JTD forms in test fixtures.

[clemensv]

Additional requirement for this issue:
Please include JSON Structure conversion support in scope as well (import to JSON Structure and export from JSON Structure), with tests and documented mapping limitations where needed.