callee_save_test.nx

callee_save_test.nx -- exercise emit_function's save_mask > 0 path. Hand-crafts a mask that claims s0 + s3 were used by regalloc, then calls emit_function and verifies the asm output contains: * prologue saves for s0 and s3 * an epilogue label .L<fn>_epi: * epilogue restores + ld ra + addi + ret * the RETURN instruction emits `j .L<fn>_epi` instead of inline Confirms mask == 0 path stays inline-return (no epilogue label present in that output -- absence check).