feat(world-gen): larger biomes, smooth height blending, capped mountains; centralize tunables in Configuration

Terrain feel: biome regions ~10x larger (Voronoi cell size 96->960); terrain height now eases across biome borders via a Voronoi distance blend (smooth, and ~25x cheaper than the old 169-sample box blur); mountains capped to ~2/3 world height and gentled (ridged amplitude/frequency lowered), snow line lowered to match.

Centralization: ~27 world-gen/render tuning constants (biome amplitudes & frequencies, climate-noise frequencies/octaves, biome-selection thresholds, blend width, cell size, filler depth, snow line, camera near plane, spawn offset, overlay scale) hoisted from scattered literals into a region-organized, fully-commented Configuration. Configuration is threaded into NoiseSource, the biomes, BiomeRegistry, BiomeMap (+Classify), TerrainSampler, WorldGenerator (+BlockAt filler depth), and Camera via Player. Defaults equal the prior values, so generation is unchanged; worldgen tests updated for the new constructor signatures.

Game/Player.cs

@@ -32,6 +32,7 @@ public sealed class Player
         this.Camera = new Camera(spawnPosition, aspectRatio)
         {
             Fov = cfg.Fov,
+            NearPlane = cfg.CameraNearPlane,
             FarPlane = (cfg.RenderDistanceHorizontal + 1) * VoxelChunk.Size,
         };
     }