v0.23.0 — Terrain-shaped settlements + non-circular continents

Continents and towns no longer come out roughly circular — both now follow the actual shape-forming process instead of a radial/convex shortcut.

Changed

• Continents are no longer roughly circular. The single-continent heightmap drops the radial distance-from-centre falloff (which forced a disk) for a per-world ocean-facing direction, an off-centre landmass seeded from two cratonic sub-plates, and a noise-warped, directional sea-frame — so coasts are ragged and the continent runs off one edge like a real margin. Multi-continent / template / hint worlds are byte-identical (archipelago/islands unchanged).
• Organic town outlines are concave, not oval (star-shaped polar curve with arms + bays); planned grid towns stay convex. The settlement RNG stream changed, so exact organic-town geometry differs from 0.22.0.

Added

• Terrain-shaped settlements. SettlementGenerator.generate(..., terrain=field) grows the footprint until it meets water or ground too high to build — a coast hugs its shore, land between lakes grows fingers, open flats spread round.
• New public world_terrain_field(world, region) derives the field from a generated map, and a TerrainField type alias. Gallery terrain-town showcase.

Full notes in CHANGELOG.md.