fix(loaders): malformed spell_slots key raises CombatantLoadError, not bare ValueError

int(k) on a non-numeric JSON key escaped the loader as an unhandled
ValueError traceback (CLI exit 1) instead of the documented exit code 2.
Route the conversion through _parse_spell_slot_keys which wraps it in a
CombatantLoadError with a clear message. Audited the other loader-side
int()/conversions: foundry.py guards every int() with isinstance/isdigit
checks and combatant.py has no other unguarded conversion.

tests/test_combatant_loader.py

@@ -5,6 +5,8 @@ from __future__ import annotations
 import json
 from typing import TYPE_CHECKING
 
+import pytest
+
 if TYPE_CHECKING:
     from pathlib import Path
 
@@ -175,6 +177,23 @@ class TestSpells:
         assert c.spells == []
 
 
+class TestSpellSlots:
+    def test_numeric_string_keys_converted(self, tmp_path: Path) -> None:
+        data = _minimal_payload(spell_slots={"1": 4, "2": 3})
+        c = load_combatant(_write(tmp_path, data))
+        assert c.spell_slots == {1: 4, 2: 3}
+
+    def test_non_numeric_key_raises_load_error(self, tmp_path: Path) -> None:
+        data = _minimal_payload(spell_slots={"one": 4})
+        with pytest.raises(CombatantLoadError, match="spell_slots keys must be integer"):
+            load_combatant(_write(tmp_path, data))
+
+    def test_float_key_raises_load_error(self, tmp_path: Path) -> None:
+        data = _minimal_payload(spell_slots={"1.5": 4})
+        with pytest.raises(CombatantLoadError, match="spell_slots keys must be integer"):
+            load_combatant(_write(tmp_path, data))
+
+
 class TestBackwardCompatAliases:
     def test_monster_load_error_is_combatant_load_error(self) -> None:
         assert MonsterLoadError is CombatantLoadError