sky(phase-2): retail-verbatim per-vertex lighting via Surface.Luminosity

Phase 2 of the sky port. Empirically confirmed from the Phase 1 dump
(ACDREAM_DUMP_SKY=1 on the live Dereth region): retail distinguishes
self-illuminated sky meshes from lit ones by the `Surface.Luminosity`
FLOAT field (0..1), NOT by the `SurfaceType.Luminous` flag bit (none of
Dereth's sky meshes have the flag set).

Observed values on the 4 currently-visible sky GfxObjs:

  GfxObj 0x010015EE (dome, 4 surfaces)    Luminosity = 1.0
  GfxObj 0x010015EF (upper cloud)         Luminosity = 0.0
  GfxObj 0x01004C36 (lower drift cloud)   Luminosity = 0.0
  GfxObj 0x01001348 (sun/moon additive)   Luminosity = 1.0

Retail uses this as an emissive coefficient in the per-vertex lighting
formula (decompiled chunk_00500000.c:7535 FUN_00508010 + chunk_00530000.c
AdjustPlanes per-vertex math):

  tint = clamp(vec3(Luminosity) + AmbColor*AmbBright
               + max(dot(N, -sunDir), 0) * DirColor*DirBright,
               0.0, 1.0)
  fragment = texture * tint

When Luminosity=1.0 the clamp saturates → full texture brightness
regardless of time of day (dome gradient preserved; sun/moon always
bright). When Luminosity=0.0 only the ambient + diffuse term drives the
tint, so clouds pick up the time-of-day ambient (purple at midnight
per AmbColor=(200,100,255)×AmbBright=0.4 ≈ (0.31,0.16,0.40); warm tan
at dusk; pale-cool at noon).

Also empirically confirmed: raw SkyObjectReplace Transparent/Luminosity
/MaxBright are in 0..100 percent range (observed 11, 15, 22, 66, 100,
and -1 sentinel). The `/100` divide in SkyDescLoader (eeae83a) is
retail-correct; `_DAT_007a1870` in the decompile must be 0.01f.

Code changes:
- src/AcDream.Core/Meshing/GfxObjSubMesh.cs: new `Luminosity` field on
  the per-submesh record (0..1, defaults to 0 for non-sky meshes).
- src/AcDream.Core/Meshing/GfxObjMesh.cs: pull Surface.Luminosity when
  building submeshes (alongside existing Translucency capture).
- src/AcDream.App/Rendering/Sky/SkyRenderer.cs:
  - SubMeshGpu gains SurfLuminosity, propagated from GfxObjSubMesh.
  - Render() pushes uAmbientColor/uSunColor/uSunDir once per frame from
    the interpolated keyframe; uEmissive once per submesh.
  - uTint uniform removed (replaced by the vTint varying computed in
    the vertex shader).
- src/AcDream.App/Rendering/Shaders/sky.vert: computes vTint per-vertex
  using the retail AdjustPlanes formula.
- src/AcDream.App/Rendering/Shaders/sky.frag: consumes vTint, drops
  uTint uniform. uLuminosity (the per-keyframe SkyObjectReplace
  override) still applied as a final scalar multiply.

Expected visual difference from Phase 1 baseline:
  - Dome gradient: IDENTICAL (Luminosity=1 saturates).
  - Sun / moon: IDENTICAL (Luminosity=1 saturates, additive blend).
  - Clouds: now tinted by time of day. Midnight → purple haze. Noon →
    pale cool. Dusk → warm tan.

Open questions (unchanged from Phase 1 doc):
  - Does the 15s LightTickSize throttling need porting? Phase 3.
  - Does FUN_00532440 (AdjustPlanes per-cell terrain relight) need
    porting for non-sky geometry to follow the sky? Phase 3.

Build + 717 tests green.

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

src/AcDream.Core/Meshing/GfxObjMesh.cs

@@ -192,13 +192,22 @@ public static class GfxObjMesh
 
             // Resolve Surface.Type flags when a DatCollection is available
             // so the renderer can split the draw into opaque and translucent
-            // passes.
+            // passes. Also capture Surface.Luminosity (self-illumination
+            // coefficient — the FLOAT field, NOT the SurfaceType.Luminous
+            // flag bit). This is the retail signal used to make the sky
+            // dome / sun / moon texture-passthrough while clouds pick up
+            // the time-of-day ambient tint (see
+            // docs/research/2026-04-23-sky-retail-verbatim.md §6).
             var translucency = TranslucencyKind.Opaque;
+            var luminosity = 0f;
             if (dats is not null)
             {
                 var surface = dats.Get<Surface>(surfaceId);
                 if (surface is not null)
+                {
                     translucency = TranslucencyKindExtensions.FromSurfaceType(surface.Type);
+                    luminosity = surface.Luminosity;
+                }
             }
 
             result.Add(new GfxObjSubMesh(
@@ -207,6 +216,7 @@ public static class GfxObjMesh
                 Indices: kvp.Value.Indices.ToArray())
             {
                 Translucency = translucency,
+                Luminosity = luminosity,
             });
         }
         return result;

src/AcDream.Core/Meshing/GfxObjSubMesh.cs

@@ -18,4 +18,25 @@ public sealed record GfxObjSubMesh(
     /// that don't supply dat access compile and pass unchanged.
     /// </summary>
     public TranslucencyKind Translucency { get; init; } = TranslucencyKind.Opaque;
+
+    /// <summary>
+    /// Self-illumination strength of the Surface (<c>Surface.Luminosity</c>
+    /// field, 0..1 fraction — NOT the <c>SurfaceType.Luminous</c> flag bit).
+    /// Retail uses this as an emissive coefficient in the per-vertex
+    /// lighting formula:
+    /// <code>
+    ///   tint = clamp(vec3(Luminosity) + AmbColor + diffuse * DirColor, 0, 1)
+    ///   fragment = texture * tint
+    /// </code>
+    /// For Dereth's sky meshes, the DOME (0x010015EE) and SUN/MOON
+    /// (0x01001348) have <c>Luminosity=1.0</c> (self-illuminated — emissive
+    /// saturates the lighting math so the baked texture always renders
+    /// at full brightness). CLOUDS (0x010015EF, 0x01004C36) have
+    /// <c>Luminosity=0.0</c> (lit by ambient+diffuse — pick up the
+    /// time-of-day tint). See
+    /// <c>docs/research/2026-04-23-sky-retail-verbatim.md</c> §6.
+    /// Defaults to 0.0 (fully lit) so non-sky meshes render through the
+    /// normal lighting path without change.
+    /// </summary>
+    public float Luminosity { get; init; } = 0f;
 }