Draft patch... just a few lines of code as proof-of-concept.
Relevant output looks like so
For the following URL:
Only works for the filter method of faceting, not the FieldCache method.
Would be simple to add though... do a binary search for the prefix in the StringIndex and start there.
Comments? Is the external API right? facet.prefix (and per-field f.cat.facet.prefix)
I had considered a more generic facet.field.terms=ca* but Erik's comment about client escaping make me realize it's simpler on both sides to just say "prefix".