sky(phase-3b): revert Phase 2 per-vertex lighting — sky meshes are UNLIT

Phase 2 added a per-vertex lighting path to the sky shader based on the
Phase 1 dump showing dome surfaces with Luminosity=1.0 and cloud
surfaces with Luminosity=0.0. Live visual verification vs retail at
MorntideAndHalf (dayFraction=0.48, user-observed 2026-04-23) disproved
the hypothesis:

  retail: clean blue sky + white clouds
  acdream: blue-green-yellow sky sweep + greyish clouds

The "sweep" is exactly the signature of per-vertex `diffuse × sunColor`
where sunColor=(250,215,151) warm gold at ~63° east: the west-facing
cloud faces get the gold tint, east-facing stay cool, and interpolation
across the mesh produces the color sweep. Retail's clean white clouds
at the same time of day means retail is NOT applying per-vertex lighting
to sky meshes.

Revised model (unlit + SkyObjectReplace modulation):
  fragment.rgb = texture.rgb * uLuminosity
  fragment.a   = texture.a   * (1 - uTransparency)

The "purple haze night / warm dusk" effect users describe from retail
comes from SkyObjectReplace per-keyframe Luminosity dimming + Transparent
fading, NOT from a shader ambient multiply. At midnight, for example,
Replace[0] dims the dome to 11% (Luminosity_raw=11) and Replace[2]
fully hides the drifting cloud (Transparent_raw=100) — so the camera
sees the dome texture at 11% × baked gradient colors, and any purple
the user perceives is baked into the dome texture's night gradient.

The retail-authoritative Surface.Luminosity flag probably feeds a
separate render path (material system? D3D emissive vs diffuse
coefficients?) that is NOT per-vertex GL lighting. A future phase can
revive it if the decompile hunt for the DayGroup selection algorithm
surfaces it.

Code change: sky.vert + sky.frag only. The C# renderer still pushes
uAmbientColor/uSunColor/uSunDir/uEmissive uniforms — they are declared
in the shaders but unused in Phase 3b. No renderer change needed; these
uniforms cost nothing and keep the port-forward path open.

Build + 717 tests green.

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

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

@@ -1,28 +1,29 @@
 #version 430 core
-// Sky mesh fragment shader — retail-verbatim composite:
+// Sky mesh fragment shader — UNLIT texture passthrough modulated by the
+// per-keyframe SkyObjectReplace.Luminosity and .Transparent overrides.
 //
-//     fragment.rgb = texture.rgb * vTint * uLuminosity + lightning_flash
+//     fragment.rgb = texture.rgb * 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.
+// uLuminosity defaults to 1.0 (no dim). A SkyObjectReplace entry with
+// Luminosity_raw=11 (11%) sets uLuminosity to 0.11 — mesh renders at
+// 11% brightness. MaxBright is min-clamped into uLuminosity by the C#
+// renderer before it reaches the shader.
+// uTransparency defaults to 0.0. Replace.Transparent_raw=100 (100%) sets
+// uTransparency to 1.0 — alpha is zeroed and the pixel discarded
+// (cloud hidden so the dome behind shows through).
 //
-// See docs/research/2026-04-23-sky-retail-verbatim.md §6 + §7.
+// See `docs/research/2026-04-23-sky-retail-verbatim.md` §6 + Phase 3b
+// rationale in sky.vert.
 
 in vec2 vTex;
-in vec3 vTint;
 out vec4 fragColor;
 
 uniform sampler2D uDiffuse;
-uniform float uTransparency;    // 0 = fully visible, 1 = fully transparent
-uniform float uLuminosity;      // 1.0 = normal; <1 dims per SkyObjectReplace
+uniform float uTransparency;
+uniform float uLuminosity;
 
-// Shared SceneLighting UBO — only need fog/flash channel for the
-// client-driven lightning strobe. Sun/ambient already baked into vTint.
+// Shared SceneLighting UBO — only fog-flash channel used (lightning).
 struct Light {
     vec4 posAndKind;
     vec4 dirAndRange;
@@ -40,16 +41,13 @@ layout(std140, binding = 1) uniform SceneLighting {
 void main() {
     vec4 sampled = texture(uDiffuse, vTex);
 
-    // Composite: texture × per-vertex lighting × per-keyframe dim.
-    vec3 rgb = sampled.rgb * vTint * uLuminosity;
+    // Unlit passthrough with per-keyframe dim.
+    vec3 rgb = sampled.rgb * uLuminosity;
 
-    // Lightning additive bump (client-side during storm keyframes).
+    // Lightning additive bump (client-driven during storm keyframes).
     float flash = uFogParams.z;
     rgb += flash * vec3(1.5, 1.5, 1.8);
 
-    // 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));