Allow roundtripping damage in folded vehicle items

[irwiss]

Summary

None

Purpose of change

Fixes #60700

Describe the solution

On folding calculate average part damage and serialize into item_var and item itself.

On unfolding get the difference between current item and old item_var damage;
Start applying it to random parts in chunks

be40dc0 also fixes a couple more MSVC warnings I missed in #60568

Note that I had need for a random-looking sequence with fixed seed - I don't need it to actually be random, just so parts "look" randomly damaged in a way that tests can snapshot (and so don't rely on global rng state) and there seems to be none existing - so I added one into rng.h/cpp

Describe alternatives you've considered

Using one of the simple hashes available for the rng sequence, but it'd do the same job but with extra steps and possible bugs.

Testing

Spawn and kill zombie granny, unfold wheelchair, parts should be damaged.

Unit test is added, could also run the unit test scenario manually

Additional context

[dseguin]

LGTM