feat(render3d): the air and the light are the hour's, not one constant apiece

Aerial perspective is a curve now. A low sun shines through far more air than a
high one and shines ALONG the ground rather than down onto it, so density, height
falloff and forward scatter all ride the sun's elevation; overcast thickens the air
and flattens the scatter, because a grey sky has no disc to scatter from. `lowsun`
falls away BELOW the horizon as well as above it, or the middle of the night gets a
dawn's haze with no dawn to justify it.

The term that was missing entirely is distance DESATURATION. Blending a saturated
green ridge toward a saturated blue noon sky leaves a saturated ridge - which is why
the same valley read as a photograph at dusk, where the fog colour happened to be a
warm grey, and as a toy at one o'clock. A surface is now pulled toward its own
luminance faster than the fog itself arrives. Measured far/near saturation at the
camp: 07:00 1.11 -> 0.89, 09:00 1.04 -> 0.93, 13:00 0.98 -> 0.89.

The grade is the hour's too - nine literals bound at the draw, written by
daylight_set now. Noon is the case worth naming: direct sun is warm-white and the
only thing filling a midday shadow is a blue sky, so noon gets a cool balance over a
blue-lifted shadow with hard contrast, and dawn and dusk the reverse. Ground R-B,
lit vs shadowed: 07:00 +42.8/+14.2 -> +48.9/+15.1, 13:00 +32.2/+14.8 -> +25.2/+2.9.
Gain is left alone deliberately: the grade is `c * gain + lift * (1 - c)`, so warming
it warms the whole frame, and warming it at noon made one o'clock yellower than seven
in the morning - the opposite of the point.

The visible sky is relit. Turning a photograph on its axis does not change what
colour it was taken at, so every sunset had a mid-morning blue overhead. An analytic
sky supplies the chroma and the photograph keeps the luminance: the cloud stays where
it is and goes orange at dusk, the zenith goes deep blue at noon, and no second sky
is shipped. It fades out under the horizon and eases off under cloud.

The ground bounce follows the ground, crossing meadow to rock at the map's treeline
instead of being one green constant everywhere including above the scree.

R3D_NOAIR=1 restores all of it, so a before-and-after comes from one binary at one
hour; it joins R3D_NOCLOUD / R3D_NOSHADOW / R3D_NOGI.

Verified on macOS OpenGL, macOS Vulkan (MoltenVK) and Windows Vulkan (RTX 3070 Ti).
Backends agree: mean difference 0.15-0.88/255 within a machine. Across machines the
ORIGINAL renderer already differed by 5.02/255 at 19:12 and this build differs by
2.80, so cross-platform variance is pre-existing and did not grow. 400 frames: GL
7.4 s before and after, VK 7.0 s before and after.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/shaders/sky.frag

@@ -59,11 +59,43 @@ vec3 moonColour(vec3 dir, float night) {
   return col * night * (1.0 - 0.9 * u_moon_haze);
 }
 
+// ---- relighting the photograph ---------------------------------------------------------------
+// The sky is ONE HDRI - a South African morning - turned on its axis so its sun disc sits where
+// the hour wants it. Turning a photograph does not change what colour it was taken at, so every
+// hour of the day was lit by the same late morning: a sunset with a mid-morning blue overhead.
+//
+// What is worth keeping from the photograph is its STRUCTURE - the cloud, the gradient, the
+// sun's own disc - and what is worth replacing is its COLOUR. So the analytic sky below supplies
+// the chroma and the photograph keeps the luminance: clouds stay exactly where they are and go
+// orange at dusk the way real ones do, and the zenith goes deep blue at noon without a second
+// photograph being shipped.
+uniform float u_sky_relight;     // 0 leaves the photograph alone; the day fades this in
+vec3 analyticSky(vec3 dir) {
+  float ct = clamp(dir.y, 0.0, 1.0);
+  float cs = clamp(dot(dir, u_sun_dir), -1.0, 1.0);
+  float lowsun = 1.0 - smoothstep(0.0, 0.45, u_sun_dir.y);
+  // Rayleigh: deep overhead, pale at the horizon where the line of sight is longest
+  vec3 base = mix(vec3(0.60, 0.70, 0.85), vec3(0.16, 0.33, 0.74), pow(ct, 0.55));
+  // the sun's warmth spreads into the sky around it, and spreads FURTHER the lower it is
+  float glow = pow(max(cs, 0.0), mix(9.0, 2.0, lowsun));
+  base = mix(base, vec3(1.0, 0.55, 0.22), glow * mix(0.22, 0.88, lowsun));
+  // and at dusk the whole horizon band warms, not only the part near the sun
+  float band = (1.0 - smoothstep(0.0, 0.32, ct)) * lowsun;
+  base = mix(base, vec3(0.96, 0.52, 0.28), band * 0.55);
+  return base;
+}
+
 void main() {
   vec4 a = u_inv_vp * vec4(v_uv * 2.0 - 1.0, 1.0, 1.0);
   vec3 dir = normalize(a.xyz / a.w - u_cam_pos);
   // level 0: the equirect seam (atan wraps) would otherwise pick the smallest mip along one column
   vec3 col = min(textureLod(u_sky, skyUV(dir), 0.0).rgb, vec3(4096.0)) * u_sky_gain;
+  if (u_sky_relight > 0.001) {
+    vec3 sk = analyticSky(dir);
+    float sl = max(dot(sk, vec3(0.2126, 0.7152, 0.0722)), 1e-4);
+    float cl = dot(col, vec3(0.2126, 0.7152, 0.0722));
+    col = mix(col, sk * (cl / sl), u_sky_relight);
+  }
   float l = dot(col, vec3(0.2126, 0.7152, 0.0722));
   col = max(mix(vec3(l), col, u_sky_sat), vec3(0.0));
   // the photograph's sky dims with the day (daylight.ludic); the night adds its own