Slice #2: skill checks + humanity / cyberpsychosis

Core tabletop mechanics the rest of the engine leans on.

SkillCheck: 1d10 + STAT + SKILL vs DV. CP RED crits — nat 10 adds another d10, nat 1 subtracts one. Opposed checks tie to the defender.

Humanity: ceiling = EMP × 10. Installing chrome rolls humanity loss; effective EMP = floor(humanity/10). Status tracks stable → at_risk → cyberpsychotic. Requirement chains block orphan cyberware options.

Cyberware v0: 8 categories, ~24 entries in data/cyberware.json.

Demo that shows the tradeoff biting:

php bin/chrome-demo.php --seed=1337 --emp=8
# Fully loaded solo ends AT_RISK (EMP 2)
```

Tests: **24/24** green at this commit.