Adds an optional *rand.Rand to combatState so a fight can be driven by a
deterministic source. Auto-resolve leaves it nil and falls through to the
package global in the same call order — behaviorally identical. Threads
the source through calcDamage and rollWeaponDamage.
Groundwork for the turn-based elite/boss engine and its timeout reaper,
which seed the rng per combat_session to make fights resumable and
replayable.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>