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.App/Rendering/Shaders/sky.frag

@@ -1,24 +1,28 @@
 #version 430 core
-// Sky mesh fragment shader — sample the object's diffuse texture with
-// the scrolled UVs from the vertex stage. Unlit: sky meshes ARE the
-// gradient (r12 §2.2), not a surface lit by the sun.
+// Sky mesh fragment shader — retail-verbatim composite:
 //
-// The per-keyframe replace override can dim the mesh (Transparent) or
-// brighten it (Luminosity); those two floats arrive as uTransparency /
-// uLuminosity uniforms.
+//     fragment.rgb = texture.rgb * vTint * uLuminosity + lightning_flash
+//     fragment.a   = texture.a   * (1 - uTransparency)
+//
+// vTint arrives from the vertex shader with retail's per-vertex lighting
+// baked in (emissive + ambient + diffuse × sun, clamped to [0,1]).
+// uLuminosity is the per-keyframe SkyObjectReplace override (0..1
+// fraction after the /100 scale in SkyDescLoader) — NOT to be confused
+// with the Surface.Luminosity that feeds uEmissive in the vertex shader.
+// uTransparency is the per-keyframe SkyObjectReplace alpha-fade.
+//
+// See docs/research/2026-04-23-sky-retail-verbatim.md §6 + §7.
 
 in vec2 vTex;
+in vec3 vTint;
 out vec4 fragColor;
 
 uniform sampler2D uDiffuse;
-uniform float uTransparency;    // 0 = fully visible, 1 = invisible
-uniform float uLuminosity;      // 1 = normal, >1 makes the mesh glow
-uniform vec4  uTint;            // per-object color tint (default white)
+uniform float uTransparency;    // 0 = fully visible, 1 = fully transparent
+uniform float uLuminosity;      // 1.0 = normal; <1 dims per SkyObjectReplace
 
-// Shared SceneLighting UBO — we only need the fog parameters to let the
-// horizon band of the sky blend smoothly into the scene's fog color at
-// the far edge, and the lightning flash to give storms their signature
-// strobe.
+// Shared SceneLighting UBO — only need fog/flash channel for the
+// client-driven lightning strobe. Sun/ambient already baked into vTint.
 struct Light {
     vec4 posAndKind;
     vec4 dirAndRange;
@@ -36,24 +40,20 @@ layout(std140, binding = 1) uniform SceneLighting {
 void main() {
     vec4 sampled = texture(uDiffuse, vTex);
 
-    // Apply tint + luminosity. Retail's SkyObjReplace.Luminosity can push
-    // above 1 to make the sun mesh brighter than its texture; r12 §2.3.
-    vec3 rgb = sampled.rgb * uTint.rgb * uLuminosity;
+    // Composite: texture × per-vertex lighting × per-keyframe dim.
+    vec3 rgb = sampled.rgb * vTint * uLuminosity;
 
-    // Lightning additive bump — makes the sky itself flash during storms.
-    // 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.
+    // Lightning additive bump (client-side during storm keyframes).
     float flash = uFogParams.z;
     rgb += flash * vec3(1.5, 1.5, 1.8);
 
-    // 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.
+    // Soft clamp. Normal frame: cap at 1.2 so emissive meshes have a
+    // small highlight margin. During a flash the ceiling relaxes so the
+    // strobe actually blows out instead of getting pinned 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;
+    float a = sampled.a * (1.0 - uTransparency);
     if (a < 0.01) discard;
     fragColor = vec4(rgb, a);
 }