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.

src/pf1e_simulator/loaders/combatant.py

@@ -221,6 +221,25 @@ def _merge_resource_grants(
     return merged
 
 
+def _parse_spell_slot_keys(raw_slots: dict[str, Json], path: Path) -> dict[int, Json]:
+    """Convert JSON ``spell_slots`` string keys to ints, failing cleanly.
+
+    JSON object keys are always strings; the ``Combatant.spell_slots`` field
+    is ``dict[int, int]``.  A non-numeric key (e.g. ``"one": 4``) is a data
+    error and must surface as a ``CombatantLoadError`` (CLI exit code 2),
+    never as a bare ``ValueError`` traceback escaping the loader.
+    """
+    parsed: dict[int, Json] = {}
+    for key, value in raw_slots.items():
+        try:
+            slot_level = int(key)
+        except ValueError as exc:
+            msg = f"spell_slots keys must be integer spell levels, got {key!r}"
+            raise CombatantLoadError(path, msg) from exc
+        parsed[slot_level] = value
+    return parsed
+
+
 def _auto_attack_ids(data: dict[str, Json], monster_id: str) -> None:
     """Auto-assign attack ids from file stem if missing."""
     attacks = data.get("attacks")
@@ -284,7 +303,7 @@ def load_combatant(path: Path) -> Combatant:
 
     raw_slots = data.pop("spell_slots", None)
     if isinstance(raw_slots, dict):
-        validate_payload["spell_slots"] = {int(k): v for k, v in raw_slots.items()}
+        validate_payload["spell_slots"] = _parse_spell_slot_keys(raw_slots, path)
 
     try:
         return Combatant.model_validate(validate_payload)