IRLowerCallArg: box scalars passed to Variant parameters

Filed by the N-lane agent, self-resolved (sole-A confirmed this session). l.append(1) — an int literal to a const v: Variant param — hit IR_UNSUPPORTED (IRLowerAddress of AN_INT_LIT). Now: hidden variant temp via the ordinary assignment path (IR_VAR_STORE owns tagging), temp address passed (variant args always pass by address). Parser's by-ref lvalue check gains a const-Variant escape at both call-arg sites; class-method param parsing now persists const-ness (mPConst -> ProcParamIsConst) on both decl paths. Resolved with [[feature-nilpy-list]].