Smooth ground scroll: redraw corridor every frame

Replaced refetch-based updates with continuous per-frame redraw.
The corridor now tracks the camera's continuous world position
(floating point, not floored to a tile) so the tile content
scrolls smoothly underneath the camera. No more 'jump by 1 tile'
at every refetch boundary.

Tradeoff: every frame draws ~65 images and does one texImage2D
upload. Browser caches the Image elements, so the cost is mainly
compositing + upload (~5MB at 6400x1280). Should still hit 30+ fps.

This is closer to how real flight sims work: the world content
scrolls continuously, not in tile-size jumps.

vendor/clouds3d.js

@@ -307,6 +307,8 @@ var groundCenterTile = { x: GROUND_BASE_X, y: GROUND_BASE_Y };
 var groundLoadedTiles = {}; // "x_y" -> true
 var groundLastCheck = 0;
 var groundUpdatePending = false;
+var groundCameraX = 0;
+var groundCameraZ = 0;
 
 function fetchTileImage(tileX, tileY) {
     return new Promise(function(resolve, reject) {
@@ -416,6 +418,44 @@ function rebuildGroundMosaic(centerTileX, centerTileY) {
     });
 }
 
+// Continuously redraw the corridor based on the live camera
+// position. Without this the corridor would shift in tile-size jumps
+// at every refetch; with it the corridor scrolls smoothly as the
+// camera flies. The browser caches the loaded Image elements so the
+// only per-frame cost is compositing + a texImage2D upload (~5MB).
+function refreshGroundMosaic() {
+    if (!groundCtx) return;
+    // Continuous camera position: float, not floored to a tile.
+    var fdx = (window.__flightDir && window.__flightDir.dx) || 0;
+    var fdz = (window.__flightDir && window.__flightDir.dz) || 1;
+    var camX = (fdx !== 0) ? groundCameraX * fdx : 0;
+    var camZ = (fdz !== 0) ? groundCameraZ * fdz : 0;
+    var centerTileX = GROUND_BASE_X + Math.floor(camX);
+    var centerTileZ = GROUND_BASE_Y + Math.floor(camZ);
+    groundCenterTile = { x: centerTileX, z: centerTileZ };
+    var halfLong = Math.floor(GROUND_CORRIDOR_LONG / 2);
+    var halfPerp = Math.floor(GROUND_CORRIDOR_PERP / 2);
+    // Clear and redraw every frame.
+    groundCtx.fillStyle = '#7c9aa1';
+    groundCtx.fillRect(0, 0, groundCanvas.width, groundCanvas.height);
+    var pending = [];
+    for (var perp = -halfPerp; perp <= halfPerp; perp++) {
+        for (var lng = -halfLong; lng <= halfLong; lng++) {
+            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 tileZ = centerTileZ + dz;
+            pending.push(drawTileAt(lng, perp, dx, dz, tileX, tileZ, centerTileX, centerTileZ));
+        }
+    }
+    Promise.all(pending).then(function() {
+        if (groundTexture && groundGl) {
+            groundGl.bindTexture(groundGl.TEXTURE_2D, groundTexture);
+            groundGl.texImage2D(groundGl.TEXTURE_2D, 0, groundGl.RGBA, groundGl.RGBA, groundGl.UNSIGNED_BYTE, groundCanvas);
+        }
+    });
+}
+
 // Called periodically from the render loop with the camera world
 // position. Re-centres the mosaic if we're approaching its edge.
 function maybeRefetchGround(cameraWorldX, cameraWorldZ, gl) {
@@ -586,10 +626,18 @@ window.clouds3dRender = function(target) {
     gl.uniform2f(s.uFlightDir, flightDir.dx, flightDir.dz);
 
     var drift = now * speed * 0.12;
-    if (performance.now() - groundLastCheck > 1500) {
-        groundLastCheck = performance.now();
-        maybeRefetchGround(drift, drift, gl);
-    }
+    // Track continuous camera world position (X and Z independently,
+    // since flight direction is independent from fetch direction).
+    var fdx2 = (window.__flightDir && window.__flightDir.dx) || 0;
+    var fdz2 = (window.__flightDir && window.__flightDir.dz) || 1;
+    groundCameraX = (fdx2 !== 0) ? drift * fdx2 : 0;
+    groundCameraZ = (fdz2 !== 0) ? drift * fdz2 : 0;
+    // Continuous scroll: redraw the corridor every frame so the camera
+    // glides smoothly through the tile content rather than jumping at
+    // integer tile boundaries. Browser caches the loaded Image
+    // elements so the only per-frame cost is compositing + a small
+    // texImage2D upload.
+    refreshGroundMosaic();
 
     gl.drawArrays(gl.TRIANGLES, 0, 3);
 };