Keeper KSM Notation Queries

From the author

The KSM notation string grammar accepted by `get_field`: the three-part `<record>/<type>/<field-path>` shape, the four field-path forms (plain, indexed, property, indexed-property), `field` vs `custom_field` vs `file` selectors, how the record segment is classified as a UID or a title, which characters the validator rejects outright, and how the returned value is masked.