Function build_primary_key_filter 

pub(crate) fn build_primary_key_filter(
    sst_metadata: &RegionMetadataRef,
    expected_metadata: Option<&RegionMetadata>,
    predicate: Option<&Predicate>,
) -> Option<CachedPrimaryKeyFilter>

Builds an encoded-primary-key filter from the supported tag predicates.

Predicates on fields, timestamps, or unsupported expression shapes are intentionally omitted. Callers use this as a pruning filter and must preserve the full predicate for authoritative filtering later in the scan.