fix(map): require single-character legend keys

src/pf1e_simulator/map.py

@@ -183,6 +183,13 @@ def load_map(path: Path) -> MapSpec:
 
     terrain = _split_rows(raw.terrain, "terrain", path)
     height, width = len(terrain), len(terrain[0])
+    multi_char = sorted(key for key in raw.legend if len(key) != 1)
+    if multi_char:
+        msg = (
+            "legend keys must be exactly one character (one char = one "
+            f"square), got: {multi_char}"
+        )
+        raise MapValidationError(path, msg)
     unknown = sorted({ch for row in terrain for ch in row if ch not in raw.legend})
     if unknown:
         msg = f"terrain chars missing from legend: {unknown}"