Poison disease implemented, various tweaks

[Plateaus]

Poison status effects
Monster poison applies a StatEffect.
Poison level is stored as a negative modifier.
It damages Body Points once per minute.
Damage declines each tick: e.g. -5 → -4 → -3 → -2 → -1.
Poison clears when the modifier reaches zero.
Disease status effects
Same declining tick mechanic.
deliberately changed disease from the original LoFP BP damage to Fatigue drain, so it behaves differently from poison.
Global periodic processing
Broke periodic poison/disease processing out of RemoveExpiredStatEffects.
UpdatePlayerTimers() now coordinates periodic effects and normal expirations.
Hooked player timers into the global background event loop so poison/disease tick even if the player isn't entering commands.
Spell/status-effect testing
Added DOTEST / doATest() as a GM testing hook.
Confirmed EffectiveStat() correctly changed Agility from 62 → 72.
Confirmed the spell expired/displayed correctly.
STAT improvements
Active effects display things like:
Agility I: +10 Agility — 55s remaining
Modified stats get an , e.g.: Agility: 72