feat(assets): planet generator pipeline — heightmap + globe from wiki data

Full terrain-to-render pipeline at tooling/planet-gen/:
- body_definition_parser: reads system index.md → body definitions
- planet_simulation: FBM + Voronoi ridges, Whittaker biome classification,
  D8 river routing, physics-driven craters (atmo × tectonics scaling)
- render_heightmap: 4096×2048 cartographic maps with hillshade
- planet_renderer: 512×512 globe with terrain UV mapping, clouds, rings
- biomes.toml: externalized color/classification tables (single source)
- scaffold_bodies: creates per-body index.md with YAML frontmatter
- batch: unattended processing with error handling, resume, determinism check

Supports all body types: temperate, arid, frozen, volcanic, barren,
oceanic, gas giant (banded + ringed), and moons (half-size, grey, cratered).

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

tooling/planet-gen/batch

@@ -0,0 +1,5 @@
+#!/bin/bash
+# Batch planet generation — process all systems unattended.
+# Usage: tooling/planet-gen/batch [--scaffold-only] [--generate-only] [--system GJ-144]
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+exec python3 "$SCRIPT_DIR/batch.py" "$@"