perf: add distance-dependent terrain mesh LOD

config.js

@@ -474,6 +474,16 @@ window.CONFIG = {
         edgeSkirtKm: 800,         // coarse outer terrain pulled past the horizon
         edgeSkirtRings: 24,       // coarse rings in the outer skirt
         edgeSkirtNoise: 0.28,     // fallback height variation if outer DEM is off
+        // Distance-dependent terrain mesh LOD: fine near the mosaic centre,
+        // coarser toward the border. The outer skirt remains the farthest LOD.
+        lod: {
+            enabled: true,
+            nearStepPx: 32,       // mesh vertex spacing near the centre
+            farStepPx: 96,        // mesh vertex spacing near the border
+            innerTiles: 8,        // fully fine area around the centre
+            outerTiles: 20        // fully coarse area toward the border
+            // lod: { enabled: false } // old uniform mesh
+        },
         outerDem: {
             enabled: true,        // false uses procedural fallback heights
             zoom: 7,              // reduced-resolution terrarium zoom for the skirt