v2.19.0: Terrain tools + lazy loading for advanced tools

- 26 new terrain tool definitions and bridge functions covering heightmaps,
  splat layers, trees, details/grass, holes, settings, grids, and more
- Lazy loading: unity_advanced_tool now derives routes from tool names
  (unity_terrain_list → terrain/list) for dynamic dispatch without restart
- Dynamic discovery: unity_list_advanced_tools queries _meta/routes from
  the Unity plugin to find tools not yet in the Node.js tool definitions
- Total: 285 tools (66 core + 219 advanced) across 25 categories
- Updated README with terrain capabilities and changelog

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