Support five encounter difficulty levels

[Daren9m]

Problem

The encounter generator exposes only the three official 2024 DMG build targets. DMs need a five-step scale for finer control.

Acceptance criteria

• The selectable scale is Trivial, Low, Moderate, High, and Extreme.
• Trivial and Extreme use documented, deterministic derived budgets around the official Low/High thresholds.
• Generation, assessment, difficulty badges/meter, saved settings, and shareable URLs support all five values.
• Existing saved data and links remain valid.
• Unit tests cover budgets, assessment boundaries, and generation at all five levels.