fix(loaders): non-list features and blank/non-string entries fail cleanly

features was popped and only re-added when it was a list, so a bare
string (or dict/null) loaded with zero feats - silent data loss. Now a
present non-list features raises CombatantLoadError, and the list
resolution adopts an accept-string-or-error contract: non-string or
blank entries raise instead of being silently dropped.

src/pf1e_simulator/loaders/combatant.py

@@ -187,13 +187,24 @@ class CombatantLoadError(Exception):
         self.reason = reason
 
 
-def _resolve_features(raw_features: list[Json], level: int) -> list[AbilitySpec]:
-    """Resolve a list of feat name strings to AbilitySpec objects."""
-    return [
-        _resolve_feat(item, level)
-        for item in raw_features
-        if isinstance(item, str) and item.strip()
-    ]
+def _resolve_features(
+    raw_features: list[Json], level: int, path: Path
+) -> list[AbilitySpec]:
+    """Resolve a list of feat name strings to AbilitySpec objects.
+
+    Data contract (accept-string-or-error): every entry must be a
+    non-blank string.  A non-string entry (``42``, ``None``, a nested
+    list) or a blank/whitespace-only string is a data error and raises
+    ``CombatantLoadError`` -- nothing is silently dropped, so a feat can
+    never disappear from a sheet without the loader failing loudly.
+    """
+    specs: list[AbilitySpec] = []
+    for item in raw_features:
+        if not isinstance(item, str) or not item.strip():
+            msg = f"features entries must be non-empty feat name strings, got {item!r}"
+            raise CombatantLoadError(path, msg)
+        specs.append(_resolve_feat(item, level))
+    return specs
 
 
 def _merge_resource_grants(
@@ -287,13 +298,19 @@ def load_combatant(path: Path) -> Combatant:
 
     _auto_attack_ids(data, monster_id)
 
-    # Resolve features (list of strings → list of AbilitySpec)
+    # Resolve features (list of strings → list of AbilitySpec).  A present
+    # but non-list value (e.g. a bare string) used to be popped and dropped
+    # silently, so the combatant lost every feat; it is now a data error.
     raw_level = data.get("level")
     level = raw_level if isinstance(raw_level, int) and not isinstance(raw_level, bool) else 1
+    had_features = "features" in data
     raw_features = data.pop("features", None)
+    if had_features and not isinstance(raw_features, list):
+        msg = f"features must be a list of feat name strings, got {raw_features!r}"
+        raise CombatantLoadError(path, msg)
     validate_payload: dict[str, object] = dict(data)
     if isinstance(raw_features, list):
-        specs = _resolve_features(raw_features, level)
+        specs = _resolve_features(raw_features, level, path)
         validate_payload["features"] = specs
         granted = _merge_resource_grants(data.get("resources"), specs)
         if granted: