Implements PF2e encounter difficulty alongside the existing D&D system.
PF2e uses creature level vs party level to derive XP, compares against
5-tier budgets (Trivial/Low/Moderate/Severe/Extreme), and adjusts
thresholds for party size. The indicator shows 4 bars in PF2e mode.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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