Wire up LTspice as a second simulation engine using mcltspice (Wine-based
LTspice runner). The backend architecture already had the ABC + factory
pattern; this connects the ltspice branch.
- Extract shared raw_to_waveform() into raw_convert.py with Protocol typing
so both ngspice and mcltspice RawFile objects work without coupling
- Add LtspiceEngine with deferred mcltspice import for graceful degradation
- Register "ltspice" in get_engine() with availability check
(mcltspice + Wine + LTspice.exe must all be present)
- Add startup validation log for LTspice availability
- Add mcltspice as optional dependency: pip install spicebook[ltspice]
- Integration tests auto-skip when LTspice is unavailable (Docker/CI)
