refactor(tooling): bump planet sim to native 1024×512, drop compute_rivers (#963)

- planet_simulation: GRID 512×256 → 1024×512. The elevation noise is
  resolution-independent (normalized coords + absolute freqs), so the
  finer grid samples the SAME terrain — features keep physical size,
  generation stays deterministic. Pixel-unit constants (gaussian sigma,
  crater radii, peak-filter window, erosion slope) scale by GRID_W/512.
  Validated: non-Sol bodies render same-world-crisper at 1024.
- Remove compute_rivers + _rivers_to_grid + the rivers/river_grid terrain
  keys: rivers are the Rust cascade's job (D8 drainage, D-208), the single
  source of river truth. The old heuristic didn't even reach the sea.
- render_heightmap: stop painting rivers onto the relief (cascade/Atlas
  overlay computed rivers instead).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tooling/planet-gen/render_heightmap.py

@@ -197,45 +197,7 @@ def _render_coastline(terrain: dict,
 
 
 # ---------------------------------------------------------------------------
-# Layer 5: Rivers
-# ---------------------------------------------------------------------------
-
-def _render_rivers(terrain: dict,
-                   rgb: np.ndarray) -> np.ndarray:
-    """
-    Draw rivers as anti-aliased polylines.
-    River list is in simulation grid coords (row, col) at GRID_H×GRID_W.
-    Scale to output pixels, draw with PIL.
-    """
-    rivers = terrain.get("rivers", [])
-    if not rivers:
-        return rgb
-
-    GRID_H, GRID_W = terrain["_grid_h"], terrain["_grid_w"]
-    scale_y = OUT_H / GRID_H
-    scale_x = OUT_W / GRID_W
-
-    # Work on a PIL image for anti-aliased line drawing
-    img = Image.fromarray((rgb * 255).clip(0, 255).astype(np.uint8), mode="RGB")
-    draw = ImageDraw.Draw(img)
-
-    river_col = RIVER_RGB
-
-    for path in rivers:
-        if len(path) < 2:
-            continue
-        # Scale grid coords to output pixels
-        pts = [(int(c * scale_x), int(r * scale_y)) for r, c in path]
-        # Line width scales with path length — longer rivers are wider.
-        # Base width doubled for readability at high output resolutions.
-        width = max(2, min(6, len(path) // 40))
-        draw.line(pts, fill=river_col, width=width, joint="curve")
-
-    return np.array(img).astype(np.float32) / 255.0
-
-
-# ---------------------------------------------------------------------------
-# Layer 6: Lat/lon grid
+# Layer 5: Lat/lon grid
 # ---------------------------------------------------------------------------
 
 def _render_grid(rgb: np.ndarray) -> np.ndarray:
@@ -435,10 +397,7 @@ def render_heightmap(body_def: dict,
     # 4: coastline
     rgb = _render_coastline(terrain, rgb)
 
-    # 5: rivers
-    rgb = _render_rivers(terrain, rgb)
-
-    # 6: lat/lon grid
+    # 5: lat/lon grid
     rgb = _render_grid(rgb)
 
     # Convert to PIL for text rendering