sky(phase-4b): clamp sky vTint at vertex + 1.0 fragment cap for retail parity

After Phase 4 landed the per-vertex lighting formula, user observed
acdream was still "a bit too bright" vs retail. Root cause:

- My Phase 4 shader deliberately left vTint unclamped so D3D-style
  overbright contributions to emissive meshes (dome has Emissive=1 → lit
  could reach 2.0 with ambient + sun) would clamp naturally at the
  framebuffer.
- But the frag cap was 1.2 (leaving "headroom for lightning flash"),
  letting dome vertices run 20% hotter than retail's per-channel 1.0.

Retail's D3D fixed-function pipeline clamps vertex lit colour at
D3DRS_COLORCLAMP=1 (default) BEFORE texture modulation. We now match:
- Clamp `vTint = clamp(lit, 0, 1)` in sky.vert so the saturate happens
  at the vertex stage, exactly like D3D.
- Drop normal-frame frag cap from 1.2 → 1.0 (the 3.0 flash relaxation
  stays so lightning strobes still visibly blow out).

Expected visual:
- Dome: identical appearance (was clamping to framebuffer 1.0 anyway),
  but pure retail-spec rendering so no sneaky 20% headroom.
- Clouds: unchanged (already < 1.0 at morning Rainy keyframe).
- Fragment flash during storm: unchanged — cap relaxes to 3.0 on flash.

Build + 733 tests green.

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

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

@@ -50,10 +50,11 @@ void main() {
     float flash = uFogParams.z;
     rgb += flash * vec3(1.5, 1.5, 1.8);
 
-    // Soft clamp. Normal frame caps at 1.2 so the D3D-style overbright
-    // from Emissive+Ambient+Diffuse at day-time saturates cleanly; during
+    // Normal-frame cap at 1.0 (retail D3D framebuffer clamps at 1.0
+    // per channel for RGBA8 output; vTint is already vertex-clamped so
+    // the only path above 1 is lightning flash additive bump). During
     // a flash the ceiling relaxes so the strobe blows out visibly.
-    float cap = mix(1.2, 3.0, clamp(flash, 0.0, 1.0));
+    float cap = mix(1.0, 3.0, clamp(flash, 0.0, 1.0));
     rgb = min(rgb, vec3(cap));
 
     float a = sampled.a * (1.0 - uTransparency);

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

@@ -61,8 +61,15 @@ void main() {
 
     // Retail per-vertex fixed-function lighting (AMBIENT=0 globally,
     // so the global ambient term drops; only light.Ambient contributes).
+    // Clamp to [0,1] at the vertex — retail's D3DRS_COLORCLAMP defaults
+    // to clamping lit vertex colours to 1.0 BEFORE texture modulate.
+    // Without this, a dome vertex (uEmissive=1) picks up ambient+diff
+    // on top of already-saturated emissive, producing > 1.5 lit values
+    // that our framebuffer cap (1.2) lets through as 20% overbright
+    // vs retail's 1.0-clamped reference. User-observed 2026-04-23.
     float diff = max(dot(worldNormal, uSunDir), 0.0);
-    vTint = vec3(uEmissive)        // material.Emissive
+    vec3 lit = vec3(uEmissive)      // material.Emissive
              + uAmbientColor        // material.Ambient(1) × light.Ambient
              + uSunColor * diff;    // material.Diffuse(1) × light.Diffuse × N·L
+    vTint = clamp(lit, 0.0, 1.0);
 }