Add a TypeGeopoint field type. The libregraph Location facet is kept as an
object (retrieval / numeric queries) and a sibling <name>_geopoint field
carries the {lat,lon} form for geo-distance / bbox / polygon queries,
uniform across bleve and OpenSearch via the shared mapping. PrepareForIndex
splices the sibling in at write time.
Build the bleve and OpenSearch index mappings from the Go struct via
reflection (json tags + per-field overrides) instead of hand-rolled
mappings and hit deserializers. New mapping package: BleveBuildMapping,
OpenSearchBuildMapping, Deserialize[T], PrepareForIndex; field decoding is
fail-soft. Mtime is typed as a date so mtime ranges are chronological on
both backends. Route CS3 facet parsing through mapping.DeserializeStringMap.
The any-valued (bleve hit) and string-valued (CS3 metadata) deserializers
share one generic fillStruct walker with a per-value setLeaf callback.