Function crdt_join_field 

pub fn crdt_join_field(
    current: &RuntimeValue,
    incoming: RuntimeValue,
) -> RuntimeValue

The state-based CRDT LATTICE JOIN of one struct field — the IDEMPOTENT sibling of crdt_merge_field: identical EXCEPT a bare-Int field takes per-field MAX (not ADD), so applying a redelivered state twice never double-counts. Counter/other fields defer to crdt_merge_field (already idempotent / LWW).