fix(assets): drop atomic writes, fix heightmap-size flag, default to 1024x512

The atomic .tmp→rename pattern caused silent FileNotFoundError on some
bodies. Removed in favour of direct writes — resume logic already handles
interrupted runs. Fixed --heightmap-size CLI flag which was silently
ignored due to Python default parameter binding. Changed default heightmap
resolution from 4096x2048 to 1024x512 (native simulation grid — no
information gain from upscaling).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tooling/planet-gen/render_heightmap.py

@@ -52,8 +52,8 @@ from biome_config import (
 # Output resolution
 # ---------------------------------------------------------------------------
 
-OUT_W = 4096
-OUT_H = 2048
+OUT_W = 1024
+OUT_H = 512
 UI_SCALE = OUT_W / 1024   # 4.0 — all pixel sizes scale with this
 
 # ---------------------------------------------------------------------------
@@ -390,13 +390,13 @@ def render_heightmap(body_def: dict,
                      render_mode: str = "cartographic",
                      chrome: bool = True) -> Image.Image:
     """
-    Render a 4096×2048 annotated equirectangular heightmap PNG.
+    Render an annotated equirectangular heightmap PNG.
 
     Parameters
     ----------
     body_def : dict   — body definition from body_definition_parser
     terrain  : dict   — terrain dict from planet_simulation.simulate()
-    out_w, out_h      — output resolution (default 4096×2048)
+    out_w, out_h      — output resolution (default 1024×512)
 
     Returns
     -------