Introduce a WorldGen module under Simulation with reusable helpers for
seed-based terrain heightmaps: PerlinNoise (deterministic 2D gradient
noise), FractalNoise (fBm over octaves), Falloff (island edge masks),
Heightmap (row-major grid with min-max normalize) and HeightmapGenerator
(settings to normalized heightmap). All deterministic from an integer
seed, allocation-free per sample, covered by xUnit tests.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
