refactor(versioning): apply cross-PR review feedback (#39977 H1/M3/M5)

Three small follow-ups surfaced by aminghadersohi's review of the
SoftDeleteMixin PR (#39977) that apply equally here:

- H1: cache _child_to_parent_registry() with functools.cache. Called
  twice per save flush; mapping depends only on import-time model
  classes, so unbounded cache is the right shape (no invalidation).
- M5: tighten _CHILD_BASELINE_HANDLERS type from dict[str, Any] to
  dict[str, Callable[[Session, Any, int], None]] via a named alias.
  Mypy now catches a future broken handler signature.
- M3/M4: explain the inline-import pattern once in the module
  docstrings of baseline.py and changes.py. Both modules use
  pylint disable=import-outside-toplevel uniformly because they
  load during init_versioning() before mappers are configured;
  the per-callsite "why" comments would just repeat the same
  reason. Module-level explanation + a hint to comment unusual
  cases is the cleaner shape.

M6 (listener placement) doesn't apply — init_versioning() already
runs inside init_app_in_ctx(). M8 (loose OpenAPI schema in
*/api.py docstrings) is real but its own change.