Introduce a dedicated Key rule (Char+ ("." Char+)*) so property
restriction keys can contain dots. Queries like
audio.artist:Motörhead or photo.cameraMake:Apple now parse.
Goal: keep the data structure aligned with the query namespace.
A driveItem exposes nested fields as dotted paths in its API
response (photo.cameraMake, location.latitude); users should be
able to query with the same path, without admin-configured slot
mappings or alias tables like Microsoft requires.
Both bleve and OpenSearch already treat dots as the nesting
separator for field paths and pass unknown keys through
unchanged — only the KQL grammar blocked dotted keys via
Char <- [A-Za-z]. The new Key rule lifts that restriction; no
compiler changes or field-alias maintenance is needed.
Values are unaffected — dots in values still parse as literal
characters.
