feat(gameplay): add Ocean biome with water generation and heightmap adjustments

- Introduce Ocean biome enum, builder, and detection logic.
- Add ocean water building to all existing biomes and modify heightmap thresholds for low mountainous areas.
- Skip cave and river generation in Ocean (and River) biomes; avoid carving water blocks.
- Comment out border blending call and update block fill logic.

src/gameplay/builders/plain_builder.cpp

@@ -27,6 +27,7 @@ void PlainBuilder::build_blocks() {
             m_blocks[Chunk::index(x, height, z)] = 1;
         }
     }
+    ocean_water_build();
 }
 
 void PlainBuilder::build_vegetation() { place_grass(); }