Fix q unit: kg/kg → g/kg (scaled ×1000 from ERA5); clarify input/output normalization space

variables.py

@@ -47,7 +47,7 @@ PL_VARIABLES: list[tuple[str, str, str]] = [
     ("t", "temperature", "K"),
     ("u", "u_component_of_wind", "m/s"),
     ("v", "v_component_of_wind", "m/s"),
-    ("q", "specific_humidity", "kg/kg"),
+    ("q", "specific_humidity", "g/kg"),  # scaled ×1000 from ERA5 kg/kg
 ]
 
 # ── Surface variables (20) ───────────────────────────────────────