Add tile borders and NS-mirrored corner labels (burned in)

User wants:
- Border around each tile so they can see where one ends
- Labels in the four corners, NS-mirrored (so N is at the
  bottom and S at the top, matching the tile's own NS axis)
- Labels burned in to the canvas so they always show the tile
  identity during flight (not just a live overlay)

Implementation:
- Thin dark border (rgba(0,0,0,0.55), 2px) via strokeRect just
  inside the tile bounds
- S at top-left, N at bottom-left (NS mirrored) in large bold
  text with white fill + black stroke for contrast
- y=NNN at top-right, x=NNN at bottom-right in smaller monospace
- All labels drawn directly on the canvas before texImage2D
  upload, so they're part of the texture the shader samples

vendor/clouds3d.js

@@ -354,27 +354,41 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
             -GROUND_TILE_SIZE, GROUND_TILE_SIZE);
         groundLoadedTiles[tileX + '_' + tileY] = true;
 
-        // Debug labels: 'N' at the image's north edge (top of canvas
-        // tile area after UNPACK_FLIP_Y_WEBGL), 'S' at south edge.
-        // Tile coords X,Y at the centre so we can verify the canvas
-        // arrangement matches the world directions.
-        groundCtx.fillStyle = 'rgba(255,255,255,0.85)';
+        // Thin border so the user can see where one tile ends and the
+        // next begins.
+        groundCtx.strokeStyle = 'rgba(0,0,0,0.55)';
+        groundCtx.lineWidth = 2;
+        groundCtx.strokeRect(canvasX + 1, canvasY + 1,
+            GROUND_TILE_SIZE - 2, GROUND_TILE_SIZE - 2);
+
+        // Burn labels into the four corners of the tile, NS-mirrored
+        // (so N ends up at the bottom and S at the top, mirroring the
+        // tile's own NS axis). User can read the tile identity
+        // directly from the rendered ground.
+        groundCtx.fillStyle = 'rgba(255,255,255,0.95)';
         groundCtx.strokeStyle = 'rgba(0,0,0,0.85)';
         groundCtx.lineWidth = 3;
-        groundCtx.font = 'bold 48px sans-serif';
-        groundCtx.textAlign = 'center';
-        // N at canvas top of tile area (image north is at top)
-        groundCtx.strokeText('N', canvasX + 128, canvasY + 56);
-        groundCtx.fillText('N', canvasX + 128, canvasY + 56);
-        // S at canvas bottom
-        groundCtx.strokeText('S', canvasX + 128, canvasY + 240);
-        groundCtx.fillText('S', canvasX + 128, canvasY + 240);
-        // Tile coords
-        groundCtx.font = 'bold 20px monospace';
-        groundCtx.strokeText('x=' + tileX + ' y=' + tileY,
-            canvasX + 128, canvasY + 140);
-        groundCtx.fillText('x=' + tileX + ' y=' + tileY,
-            canvasX + 128, canvasY + 140);
+        groundCtx.textAlign = 'left';
+        groundCtx.font = 'bold 40px sans-serif';
+        // S at top-left (image south after Y mirror is at canvas top)
+        groundCtx.strokeText('S', canvasX + 12, canvasY + 44);
+        groundCtx.fillText('S', canvasX + 12, canvasY + 44);
+        // N at bottom-left (image north at canvas bottom)
+        groundCtx.strokeText('N', canvasX + 12, canvasY + 244);
+        groundCtx.fillText('N', canvasX + 12, canvasY + 244);
+        // Tile coordinates, top-right and bottom-right corners
+        groundCtx.textAlign = 'right';
+        groundCtx.font = 'bold 18px monospace';
+        // y at top-right
+        groundCtx.strokeText('y=' + tileY,
+            canvasX + GROUND_TILE_SIZE - 12, canvasY + 28);
+        groundCtx.fillText('y=' + tileY,
+            canvasX + GROUND_TILE_SIZE - 12, canvasY + 28);
+        // x at bottom-right
+        groundCtx.strokeText('x=' + tileX,
+            canvasX + GROUND_TILE_SIZE - 12, canvasY + GROUND_TILE_SIZE - 12);
+        groundCtx.fillText('x=' + tileX,
+            canvasX + GROUND_TILE_SIZE - 12, canvasY + GROUND_TILE_SIZE - 12);
 
         return true;
     }).catch(function() {