Mechanical `gofmt -w ./internal ./cmd`. Mostly struct-field realignment that
had drifted, plus a few trailing-newline fixes. No behaviour change — gofmt is
semantics-preserving, and build/vet/test are green either side.
Split out from the code-review fixes that follow so those stay reviewable
instead of hiding inside a wall of realignment.
- 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.
