Mirror tile content across NS axis (image + N/S labels only)

User feedback: the corridor arrangement is correct (lng=+12
at screen top in N flight), but the per-tile image and labels
were upside down relative to expectations.

- Image drawing: changed from 180° rotation (-TILE, -TILE on
  both axes) to Y-only flip (TILE, -TILE on Y). Image top
  (north) now lands at canvas bottom; image bottom (south) at
  canvas top. X axis is preserved so image east is still on the
  right within each tile.
- N/S labels: swapped so N is at canvas bottom (= image north)
  and S at canvas top (= image south).

vendor/clouds3d.js

@@ -346,8 +346,11 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
         // (canvasX + 256, canvasY + 256). After this, image north
         // (high lat) is at canvas bottom and image south at canvas
         // top; image east at canvas right, image west at canvas left.
-        groundCtx.drawImage(img, canvasX + GROUND_TILE_SIZE, canvasY + GROUND_TILE_SIZE,
-            -GROUND_TILE_SIZE, -GROUND_TILE_SIZE);
+        // Flip image only on the Y axis (NS mirror). Image top
+        // (north) -> canvas bottom, image bottom (south) -> canvas
+        // top. X axis (EW) is preserved.
+        groundCtx.drawImage(img, canvasX, canvasY + GROUND_TILE_SIZE,
+            GROUND_TILE_SIZE, -GROUND_TILE_SIZE);
         groundLoadedTiles[tileX + '_' + tileY] = true;
 
         // Debug labels: 'N' at the image's north edge (top of canvas
@@ -359,12 +362,13 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
         groundCtx.lineWidth = 3;
         groundCtx.font = 'bold 48px sans-serif';
         groundCtx.textAlign = 'center';
-        // N at canvas top of tile area (= north in image)
-        groundCtx.strokeText('N', canvasX + 128, canvasY + 56);
-        groundCtx.fillText('N', canvasX + 128, canvasY + 56);
-        // S at canvas bottom of tile area
-        groundCtx.strokeText('S', canvasX + 128, canvasY + 240);
-        groundCtx.fillText('S', canvasX + 128, canvasY + 240);
+        // N at canvas bottom of tile area (image north is at canvas
+        // bottom because image is Y-flipped).
+        groundCtx.strokeText('N', canvasX + 128, canvasY + 240);
+        groundCtx.fillText('N', canvasX + 128, canvasY + 240);
+        // S at canvas top
+        groundCtx.strokeText('S', canvasX + 128, canvasY + 56);
+        groundCtx.fillText('S', canvasX + 128, canvasY + 56);
         // Tile coords
         groundCtx.font = 'bold 20px monospace';
         groundCtx.strokeText('x=' + tileX + ' y=' + tileY,