Align motion, look and corridor on +Z (forward)

Camera was moving in (1, 0.7) but looking mostly in +Z — 55
deg between them meant tiles collected along motion weren't in
the user's forward view. Now:
- ro = (0, 2.2, drift) \u2014 motion purely in +Z
- GROUND_TRAVEL_COS/SIN = (0, 1) \u2014 corridor runs along +Z
- rd = mostly forward +Z, with downward tilt for the ground view

Everything (motion, look, tile collection) is now on the same
+Z axis. Tiles appear directly in front of the camera as it
moves forward, no more 45-55 deg offset.

vendor/clouds3d.js

@@ -139,7 +139,7 @@ var FRAG_SRC = [
         'float drift = uTime * uSpeed * 0.45;',
         // Fixed altitude well above the cloud ceiling (y=1.5).
         'float camY = 2.2;',
-        'vec3 ro = vec3(drift, camY, drift * 0.7);',
+        'vec3 ro = vec3(0.0, camY, drift);',
         // Ray direction: tilt more downward so the visible ground fits
         // within the mosaic. At uv.y = 0 the ray goes ~26° below
         // horizontal, putting the visible ground at tGround < 5
@@ -271,12 +271,10 @@ function init(canvas) {
 var GROUND_TILE_URL = 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/';
 var GROUND_TILE_Z = 11;
 var GROUND_TILE_SIZE = 256;
-// Camera travels at uTime * uSpeed * 0.45 in +X (and +X*0.7 in +Z).
+// Camera travels in +Z (forward) and looks in +Z (also forward).
-// Travel angle: atan2(0.7, 1) ~ 35 deg. Normalised: cos=0.82, sin=0.57.
+// The corridor is aligned with both motion and look direction.
-// The corridor is ROTATED to follow the actual travel direction so
+var GROUND_TRAVEL_COS = 0.0;      // cos(90 deg)
-// tiles aren't wasted in regions the camera never reaches.
+var GROUND_TRAVEL_SIN = 1.0;      // sin(90 deg)
-var GROUND_TRAVEL_COS = 0.82;     // cos(35 deg), = 1 / sqrt(1 + 0.49)
-var GROUND_TRAVEL_SIN = 0.57;     // sin(35 deg), = 0.7 / sqrt(1.49)
 var GROUND_CORRIDOR_LONG = 15;    // tiles along the travel direction
 var GROUND_CORRIDOR_PERP = 5;     // tiles perpendicular to travel
 var GROUND_BUFFER_LONG = 2;       // refetch when this close to forward edge
@@ -369,6 +367,8 @@ function maybeRefetchGround(cameraWorldX, cameraWorldZ, gl) {
     var cameraTileX = GROUND_BASE_X + Math.floor(cameraWorldX);
     var cameraTileY = GROUND_BASE_Y + Math.floor(cameraWorldZ);
     // Project camera offset onto the corridor's rotated axes.
+    // With GROUND_TRAVEL_COS/SIN = (0, 1) the corridor runs along
+    // +Z (camera's motion + look direction).
     var dx = cameraTileX - groundCenterTile.x;
     var dy = cameraTileY - groundCenterTile.y;
     var projLong = dx * GROUND_TRAVEL_COS + dy * GROUND_TRAVEL_SIN;