chore(assets): fix ruff lint warnings in planet-gen scripts

Remove unused imports (os, sys, math, ImageFilter, gaussian_filter,
MAX_BIOME_ID) flagged by ruff F401.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tooling/planet-gen/biome_config.py

@@ -13,7 +13,6 @@ Usage:
     )
 """
 
-import os
 from pathlib import Path
 
 try:

tooling/planet-gen/body_definition_parser.py

@@ -41,7 +41,6 @@ import logging
 import math
 import os
 import re
-import sys
 from pathlib import Path
 from typing import Optional
 

tooling/planet-gen/render_heightmap.py

@@ -37,16 +37,14 @@ Usage:
     img.save("GJ144d_heightmap.png")
 """
 
-import math
 import numpy as np
-from PIL import Image, ImageDraw, ImageFilter, ImageFont
-from scipy.ndimage import gaussian_filter, binary_dilation
+from PIL import Image, ImageDraw, ImageFont
+from scipy.ndimage import binary_dilation
 
 from biome_config import (
     BIOME_PALETTE as _BIOME_PALETTE_CFG,
     RIVER_RGB as _RIVER_RGB_CFG,
     COAST_RGB as _COAST_RGB_CFG,
-    MAX_BIOME_ID,
     build_biome_rgb,
 )
 
@@ -451,7 +449,7 @@ def render_heightmap(body_def: dict,
 # ---------------------------------------------------------------------------
 
 if __name__ == "__main__":
-    import sys, json, time, os
+    import sys, json, time
 
     if len(sys.argv) < 2:
         print("Usage: python3 render_heightmap.py body_def.json [--small]")