feat(spells): add spell system with 10 combat spells, cast_spell action

Spells are SpellSpec dataclasses with typed effects: DamageEffect
(dice + types, half-on-save), ConditionEffect (condition lookup +
save negates), HealEffect (capped at hp_max), BuffEffect (transient
StatModifiers). Range categories (personal/touch/close/medium/long)
scale with caster level via spell_range_ft. JSON loader
(load_spell/load_spell_registry) reads data/spells/*.json.

The engine integrates spells via Action(kind='cast_spell') and
_cast_spell: range check, line-of-effect gate, save resolution
(resolve_save), effect application (_apply_spell_effects with DR
via _apply_dr_for_types), transcript logging. The spell_registry
is passed to CombatEngine; Combatant.spells holds known spell names.

- spells.py: SpellSpec, 4 effect types, SpellRange/SaveType literals,
  spell_range_ft, JSON loader (uses Json type from foundry.py)
- conditions.py: CONDITIONS_BY_NAME registry for condition lookup
- combat.py: cast_spell Action kind, _execute_cast_spell dispatch,
  _cast_spell (range/LoE/save/effects/log), _apply_spell_effects,
  _apply_dr_for_types, _save_label helper, SpellSpec/BuffEffect/etc
  imports, spell_registry parameter on CombatEngine
- models.py: spells field on Combatant
- data/spells/: 10 spells (Magic Missile, Burning Hands, Fireball,
  Lightning Bolt, Acid Arrow, Cure Light/Moderate Wounds, Hold Person,
  Fear, Bull's Strength)
- tests/test_spells.py: 29 tests (loading, range, damage/save/condition/
  heal/buff/DR, out-of-range, unknown spell, dispatch, DC scaling)
- pyproject.toml: PLR0913 ignore for combat.py (CombatEngine.__init__)
- README.md: spells.py architecture section, Sorts removed from
  non-modeled list, 336 tests

data/spells/fireball.json

@@ -0,0 +1,17 @@
+{
+  "name": "Fireball",
+  "level": 3,
+  "school": "evocation",
+  "range": "long",
+  "save": "ref",
+  "base_dc": 10,
+  "effects": [
+    {
+      "type": "damage",
+      "formula": "6d6",
+      "types": ["fire"],
+      "half_on_save": true
+    }
+  ],
+  "description": "A glowing bead streaks from you and bursts into a 20-foot radius explosion of fire, dealing 6d6 points of fire damage. Reflex save for half."
+}