Avoid post-filtering for plain label index queries

When a label query has no group or count constraint, the index result is
exact — the label value and account are stored verbatim in the index.
Post-filtering adds no correctness benefit but triggers a fallback that
re-evaluates the full predicate tree, including branches like comment:
that hit NoteDB for every candidate change.

Add a new predicate type that trusts the index result directly, used
whenever no group membership or vote count needs to be verified. The
same applies to magic label queries. External groups continue to use
post-filtering unchanged.

Release-Notes: Label queries without group or count constraints no longer trigger post-filtering, avoiding unnecessary NoteDB reads.
Change-Id: I75d1c2896fe69ef1142b1acae6f39a004e3d2abc