v0.21.0 — Grid streets

Towns can now be planned, not just organic. A new layout knob lays a real geometric street grid aligned to the town's long axis, with matching rectangular blocks.

Added

• SettlementConfig.layout — "organic" (default) or "grid". In grid mode a town gets:
  • a geometric street grid aligned to the footprint's principal (long) axis — two families of parallel thoroughfares clipped to the footprint, the central line of each marked "main";
  • gates where the main streets pierce the perimeter (plus a harbour gate when coastal);
  • grid-aligned lots — blocks are bisected along the grid axes, so buildings come out rectangular and street-aligned;
  • walls that splice the mid-edge grid gates into the wall ring as real gatehouse gaps.
• New preset grid_city and a grid-city gallery showcase.
• New reusable geometry primitive clip_line_to_convex (Liang–Barsky line ↔ convex-polygon clipping).

Compatibility

• The default (layout="organic") output is byte-identical to v0.20.0 — all grid logic is gated behind the new mode.
• Purely additive: layout serialises via to_dict / json_schema (new enum-field support in SettlementConfig). No public-API names added.

Full changelog: 7d18d4e...v0.21.0