fix: resolve memory leak in CPU destructors

[mariusmm]

Fix double-delete bug where m_qk was allocated in base class CPU constructor but deleted in derived class destructors (CPURV32/CPURV64).

Changes:

• Move m_qk deletion from derived class destructors to base class CPU destructor where it was originally allocated
• Add null pointer checks before all delete operations in destructors
• Set pointers to nullptr after deletion for safety
• Add comments explaining cleanup responsibility

This fixes:

• Double-delete bug that could cause undefined behavior
• Potential memory leaks from improper cleanup order
• Missing null pointer safety checks

Files modified:

• inc/CPU.h: Changed base class destructor from =default to proper implementation
• src/CPU.cpp: Added base class destructor to delete m_qk
• src/RV32.cpp: Removed m_qk deletion, added null checks
• src/RV64.cpp: Removed m_qk deletion, added null checks