feat: add D&D 5.5e encounter difficulty system

[CozKun]

Summary

Add a separate D&D 5.5e RPG system for the revised encounter-building rules while leaving the existing D&D 5e system unchanged.

This is a focused, current-main follow-up to #323 by @Tarrayna. It preserves the original proposal's intent while correcting the revised threshold table and covering several edge cases. It intentionally excludes package, lockfile, version, changelog, and release-tooling changes.

Changes

• Add the revised Low, Moderate, and High XP thresholds for character levels 1–20.
• Calculate encounter difficulty from raw monster XP totals without creature-count multipliers.
• Preserve explicit creature XP values of 0 instead of falling back to CR-derived XP.
• Preserve bestiary XP lookup and CR-derived XP fallback when XP is absent.
• Treat encounters without a valid party as Trivial instead of matching zero-valued High thresholds.
• Map Low, Moderate, and High to the existing encounter UI color classes.
• Reuse the existing CR-to-XP table rather than duplicating it.
• Register the system in the existing enum-driven XP System dropdown as D&D 5.5e.

The threshold values come from the official revised XP Budget per Character table.

Compatibility

• Existing D&D 5e calculations are unchanged.
• Existing statblocks, creature XP values, bestiary lookup, CR fallback, and encounter UI remain supported.
• Existing saved settings continue to use their current RPG system unless the new option is selected.

Validation

• npm ci
• npx tsc --project tsconfig.json --noEmit
• npm run build
• Runtime assertions for all 20 threshold rows, Low/Moderate/High boundaries, raw multi-creature XP, empty-party behavior, explicit xp: 0, and CR fallback
• Manual testing in Obsidian, including encounter builder and tracker behavior

All checks passed.