[gbeane]

Moves the aging study's batch and tester from the subject to the session and bumps the extension to 0.3.0.

• New JaxAgingStudyMetaData (extends LabMetaData, default name jax_aging) with optional batch and tester text fields. Add it with nwbfile.add_lab_meta_data(...).
• JaxAgingSubject keeps cfi, cfi_norm, pll and diet.
• Breaking: batch and tester are removed from JaxAgingSubject. They shipped in 0.2.0, but no files have been written with them.
• README and CHANGELOG updated; spec regenerated from create_extension_spec.py; 9 new tests (53 total), including one file that carries both a JaxAgingSubject and the session metadata and validates.
• uv.lock only bumps the package version.

After merge: tag 0.3.0 to publish to PyPI.