return_pc for each frame belongs on the caller's program_counter (the
resume address when this routine exits). varnum is the store variable
that receives the return value, stored as return_addr on the frame
itself. Also handle flags bit 4 (discard result → return_addr=None).
Verifies complete save/restore pipeline by generating save data with
QuetzalWriter and restoring it with QuetzalParser. Tests cover:
- Basic round-trip with memory, stack, and PC restoration
- Multiple nested call frames
- Preservation of unchanged memory bytes (run-length encoding)
- Empty stack (no routine frames)
Each test confirms that state modified after save is correctly
restored to original values from the save data.
