- Enforce per-level Mage known-spell cap in handleSpellsLearn; surface
spellbook budget in renderSpellsList.
- Mage level-up DM now nudges with "Spells available to learn: N" via
extracted buildLevelUpMessage.
- Extract halveSavedDamage and enemySpellSaveMod for clarity; document
single-target AoE limitation in applySpellDamageSave.
- Add tests: mage learn cap, prepare flow, AoE behavior, spell save
rounding, spells migration.
