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/GameWindow.cs

@@ -886,7 +886,18 @@ public sealed class GameWindow : IDisposable
         // Seed WorldTime to noon so outdoor scenes aren't pitch-black before
         // the server sends its first TimeSync packet (offline rendering in
         // particular never receives one).
-        WorldTime.SyncFromServer(AcDream.Core.World.DerethDateTime.DayTicks * 0.5);
+        //
+        // "Noon" here means sun at zenith — dayFraction = 0.5. Because
+        // DerethDateTime applies a +7/16 offset (tick 0 = Morntide-and-Half,
+        // hour 8 of 16), we need raw ticks = 476.25 (one hour past tick 0 =
+        // Midsong / Hour 9, which is what retail considers noon).
+        //
+        // Using `DayTicks * 0.5 = 3810` WOULD be correct if the offset were
+        // zero, but with our 3333.75-tick shift it lands on dayFraction
+        // 0.9375 — that's Gloaming-and-Half (sunset, nearly midnight),
+        // producing a dim orange sky with the sun below the horizon until
+        // TimeSync arrives.
+        WorldTime.SyncFromServer(AcDream.Core.World.DerethDateTime.DayTicks / 16.0); // = 476.25 = Midsong (noon)
 
         // Build the terrain atlas once from the Region dat.
         var terrainAtlas = AcDream.App.Rendering.TerrainAtlas.Build(_gl, _dats);
@@ -3504,10 +3515,12 @@ public sealed class GameWindow : IDisposable
         var kf = WorldTime.CurrentSky;
         var atmo = Weather.Snapshot(in kf);
         var fogColor = atmo.FogColor;
-        // Clamp to 0..1 — keyframes may store over-1 values (retail uses the
-        // dir-bright scalar pre-multiplied into color) and GL's ClearColor
-        // will silently accept them, but some drivers interpret > 1 as
-        // "bright clamp", producing ugly pink/green frames.
+        // Clear to fog color (horizon haze) so if sky meshes have alpha
+        // gaps or don't cover the full view, the "missing" area reads as
+        // distant haze, not as pitch-black. Fog color is clamped to 0..1
+        // since keyframes may pre-multiply DirBright and produce over-1
+        // values that some drivers interpret as "bright clamp" (pink/green
+        // frames).
         _gl!.ClearColor(
             System.Math.Clamp(fogColor.X, 0f, 1f),
             System.Math.Clamp(fogColor.Y, 0f, 1f),