Flat net ability mods aren't equal *effective* power: a +1 in a stat a
build uses beats a +1 in a dump stat. Add dnd_race_balance.go, which
scores each race's mods against a 60/40 blend of per-class combat stat
priorities and class-independent non-combat utility (zone locks,
expedition harvest, skill checks, haggling).
Retune all races so their mean score across playable classes lands
within ±0.5 of the Standard Human baseline (6.0); best-fit/worst-fit
spread is kept as intentional race identity. TestRaceBalance logs the
report and asserts the rule.
