fix(sky): scale keyframe Luminosity/Transparent/MaxBright from percent → fraction

Retail's Region dat stores SkyObjectReplace.Luminosity / Transparent /
MaxBright as percentages in the 0..100 range. Our shader expects
fractions in 0..1. We were passing raw values (luminosity up to 100)
straight into the sky fragment shader's rgb-multiplier:

    rgb = sampled.rgb * uTint.rgb * uLuminosity;

At the "Sunny" DayGroup's noon keyframes (verified via live diag),
Luminosity = 100 → shader multiplied the cloud texture RGB by 100 →
min(rgb, vec3(1.2)) clamped all channels to 1.2 → pure white sky.

Also gave the dawn/dusk purple sky effect on top of the pale texture.

Fix: SkyDescLoader.ConvertTimeOfDay divides Luminosity, Transparent
and MaxBright by 100 when loading each SkyObjectReplace. The Rotate
field stays as degrees (values like 270° are genuine headings, not
percentages).

Transparent was accidentally surviving via a 0..1 clamp downstream,
but we fix it for consistency and so brightness-attenuating values
in the 0..99 range (partial fade during dawn/dusk) work correctly
instead of rounding to full-transparent.

WorldBuilder's SkyboxRenderManager does NOT apply these fields at
all — that's why they never hit this bug. Our port applies them for
per-keyframe day-night fades, so we needed the unit conversion.

Also picked up in this commit (incidental, already running):
 - Sky render: per-submesh blend mode from TranslucencyKind.Additive
   for sun/moon-style self-bright objects (Additive bit 0x10000).
   Luminous flag 0x40 intentionally NOT mapped to additive — that
   flag is on the sky dome + cloud sheets and making them additive
   produced the previous "fully white" iteration of this bug.
 - ToD default seed: DayTicks/16 (Midsong = hour 9 = true noon)
   instead of DayTicks*0.5 which landed on Gloaming-and-Half (sunset)
   due to DerethDateTime's +7/16 day-fraction offset. Pre-TimeSync
   view now correctly starts at noon.
 - Lightning flash: brighter white-blue (vec3(1.5,1.5,1.8)) instead
   of dim grey; ceiling relaxed during flash so the strobe actually
   blows out. Cadence (strike intervals, decay) unchanged.
 - Saved docs/research/2026-04-21-sky-deep-audit.md with the
   decompile+ACE+ACME+WorldBuilder research done to corner this bug.

Open follow-up (not fixed here): sky clouds are white at noon /
don't get the dusk/night purple tint. Our sky shader is fully unlit
— doesn't apply sun/ambient directional light like the terrain
shader does. AmbientColor in the keyframe data carries the right
tint (purple at midnight, magenta at dusk) but we pass
uTint = Vector4.One instead of the keyframe value. Next commit will
wire directional-sun + ambient into sky.frag so cloud meshes pick
up the time-of-day color.

All 717 tests green. User-confirmed: sky colors are now "much
better" after this change (previously fully white).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/Shaders/sky.frag

@@ -41,9 +41,17 @@ void main() {
     vec3 rgb = sampled.rgb * uTint.rgb * uLuminosity;
 
     // Lightning additive bump — makes the sky itself flash during storms.
-    rgb += uFogParams.z * vec3(0.5, 0.5, 0.55);
+    // Retail's lightning is a near-white strobe; a dim grey bump doesn't
+    // read as lightning. Keep a faint blue tint so it still feels electric
+    // rather than pure-white daylight.
+    float flash = uFogParams.z;
+    rgb += flash * vec3(1.5, 1.5, 1.8);
 
-    rgb = min(rgb, vec3(1.2));  // soft clamp to let luminosity over-bright mildly
+    // Soft clamp to let Luminosity/flash slightly over-bright. During a
+    // lightning flash, raise the ceiling so the strobe actually blows out
+    // instead of getting capped mid-rise.
+    float cap = mix(1.2, 3.0, clamp(flash, 0.0, 1.0));
+    rgb = min(rgb, vec3(cap));
 
     float a = sampled.a * (1.0 - uTransparency) * uTint.a;
     if (a < 0.01) discard;