Partial refetch: only draw new tiles, don't clear canvas

With BUFFER_LONG = 1, refetch fires every ~1 tile of motion (every
~8 sec at default speed). With BUFFER = 1 the center shifts by
~1 tile per refetch, so only ~1 net new tile needs fetching each
time (the one at the very front of the new corridor).

Switch to partial refetch:
- Don't clear the canvas before redrawing
- Skip tiles already in groundLoadedTiles (they're still on the
  canvas at the right position from a prior refetch)
- Only newly-positioned tiles are drawn (the ones in the new
  front of the corridor, ahead in flight direction)

Effect: user sees tiles appearing at the front of the corridor in
the direction of travel, not the whole canvas flashing every 8 sec.
With N flight, fetch activity shows up at the top of the screen
(where lng=+12 lands after the axis swap), confirming the flight
direction is correct.

vendor/clouds3d.js

@@ -379,12 +379,12 @@ function rebuildGroundMosaic(centerTileX, centerTileY) {
     groundCenterTile = { x: centerTileX, y: centerTileY };
     var halfLong = Math.floor(GROUND_CORRIDOR_LONG / 2);
     var halfPerp = Math.floor(GROUND_CORRIDOR_PERP / 2);
-    // Clear with fog colour so we don't get ghosting from old tiles
+    // Partial refetch: don't clear the canvas. Tiles already loaded
-    // at wrong canvas positions. The shader's edgeFade makes OOB
+    // (from a previous refetch) stay visible. Only new tiles are
-    // samples blend smoothly to uBg, so the brief fog state during
+    // drawn on top, so fetch activity shows up at the front of the
-    // tile fetch is invisible.
+    // corridor (in flight direction) instead of flashing the whole
-    groundCtx.fillStyle = '#7c9aa1';
+    // canvas. The drawTileAt call hashes by (tileX, tileY) so already-
-    groundCtx.fillRect(0, 0, groundCanvas.width, groundCanvas.height);
+    // loaded tiles are skipped.
     var promises = [];
     for (var perp = -halfPerp; perp <= halfPerp; perp++) {
         for (var lng = -halfLong; lng <= halfLong; lng++) {
@@ -393,6 +393,11 @@ function rebuildGroundMosaic(centerTileX, centerTileY) {
             // camera's actual path, not just the +X axis.
             var dx = Math.round(lng * GROUND_TRAVEL_COS - perp * GROUND_TRAVEL_SIN);
             var dz = Math.round(lng * GROUND_TRAVEL_SIN + perp * GROUND_TRAVEL_COS);
+            var tileX = centerTileX + dx;
+            var tileY = centerTileY + dz;
+            // Skip tiles already loaded (still drawn at the right canvas
+            // position from a previous refetch or the initial load).
+            if (groundLoadedTiles[tileX + '_' + tileY]) continue;
             promises.push(drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY));
         }
     }