Allow consumers to hook into the autosave save path

Split out from the coding-standards cleanup (#147), which removed a `// TODO: Allow hook into autosave` note so it could be tracked here.

In `save_post_metadata()`, metadata saving is skipped while `DOING_AUTOSAVE` unless the group opts in via its `autosave` flag. This issue tracks adding a hook/filter so consumers can run their own logic during autosave, rather than the current all-or-nothing per-group flag.

Start by reading save_post_metadata() and the existing DOING_AUTOSAVE and autosave-flag handling. Trace how metadata groups are saved, then define and verify a hook/filter that lets consumers run their own autosave logic without retaining the current all-or-nothing behavior.

Written by the indexing model from the issue text.