chore(sky): post-merge cleanups — CullFace save/restore + stale comments

Three small hygiene items flagged by external code-review reports
during the sky/weather investigation:

1. CullFace state leak in SkyRenderer.RenderPass.
   Disabled CullFace at the start of the sky pass without restoring it
   on exit. Benign today — the global convention in this codebase is
   CullFace=off and subsequent renderers (InstancedMeshRenderer,
   StaticMeshRenderer) explicitly enable on entry / disable on exit —
   but a future caller assuming culling stays on across the sky pass
   would have silently broken. Wrap with an IsEnabled save / Enable
   restore using TextRenderer.cs's pattern.

2. Stale comment in SubMeshGpu.SurfTranslucency doc.
   Said "the shader multiplies output alpha by (1 - x)". After commit
   97fc1b5 the shader uses translucency DIRECTLY as opacity per retail
   D3DPolyRender::SetSurface at 0x59c7a6 (decomp 425255-425260).
   Updated to reflect the current formula.

3. Stale comment in sky.frag header.
   Said "fragment.a = texture.a × (1 - uTransparency) × (1 - uSurfTranslucency)".
   Updated to "× uSurfTranslucency" with citation.

Not addressed: Report 2's "uLuminosity declared but never referenced"
claim. Verified false — the uniform was already removed; the only
remaining uLuminosity references are in comments documenting the
historical removal (sky.frag header line 13-14 explicitly says
"removed 2026-04-26"). Report 2 was reading stale content.

1314 tests pass.

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

@@ -3,7 +3,9 @@
 // D3D fixed-function:
 //
 //   fragment.rgb = texture.rgb × vTint + lightning_flash
-//   fragment.a   = texture.a   × (1 - uTransparency) × (1 - uSurfTranslucency)
+//   fragment.a   = texture.a   × (1 - uTransparency) × uSurfTranslucency
+//                 (uSurfTranslucency is OPACITY directly per retail's
+//                  D3DPolyRender::SetSurface at 0x59c7a6, NOT 1-x)
 //
 // vTint arrives from the vertex shader with retail's per-vertex
 // lighting formula baked in (Emissive + lightAmbient + lightDiffuse ×