refactor(versioning): purge dataset_snapshots dead code + fix get_version bug

The full-Continuum spike (ADR-004 revised) replaced the JSON-snapshot
restore path with Continuum's native Reverter and removed the
``dataset_snapshots`` / ``dashboard_snapshots`` tables from the
migration chain. Seven VersionDAO methods and two module-level
helpers that read/wrote those tables stayed in the code anyway and
went unused — dead code that looked live.

Worse, ``VersionDAO.get_version`` still read from
``dataset_snapshots`` in its SqlaTable branch. On any environment
where the snapshot tables don't exist (current production behavior),
``GET /api/v1/dataset/<uuid>/versions/<version_uuid>/`` raised
``OperationalError``. The branch is rewritten to read column and
metric state from Continuum's child shadow tables
(``table_columns_version`` / ``sql_metrics_version``) via the
existing ``_shadow_rows_valid_at`` helper.

Deleted:
- ``_deserialize_snapshot_value`` (module helper)
- ``_coerce_snapshot_list`` (module helper)
- ``RESTORE_EXCLUDE_FIELDS`` (constant — only referenced by deleted code
  and a docstring)
- ``VersionDAO._restore_dataset_children``
- ``VersionDAO._parse_slice_ids_json``
- ``VersionDAO._apply_dashboard_slices``
- ``VersionDAO._restore_dashboard_children``
- ``VersionDAO._apply_snapshot_children``

The corresponding ~17 unit tests in
``tests/unit_tests/daos/test_version_dao.py`` are removed alongside.

Stale docstring references in ``versioning/changes.py`` and
``versioning/diff.py`` that pointed at the retired snapshot tables are
also cleaned up.

Also strips an 8-line comment block in ``restore_version`` that
duplicated the docstring of ``_stamp_audit_fields_for_restore``.

Net: −290 lines from ``daos/version.py``; a production-shape bug
fixed; dead code that looked live is gone.