refactor(N.3): thread isAdditive + substitute 5 decode methods with WB TextureHelpers

Task 2 — isAdditive threading:
SurfaceDecoder.DecodeRenderSurface now accepts isAdditive parameter.
A8/CUSTOM_LSCAPE_ALPHA format splits:
- isAdditive=true:  R=G=B=A=val (terrain alpha, additive entity textures)
- isAdditive=false: R=G=B=255, A=val (non-additive entity textures)
TextureCache passes surface.Type.HasFlag(SurfaceType.Additive).
TerrainAtlas passes isAdditive:true (alpha masks always replicate).
Aligns with WB ObjectMeshManager dispatch logic.

Task 3 — WB body substitution + new formats:
INDEX16, P8, A8R8G8B8, R8G8B8, A8 now delegate to
TextureHelpers.FillIndex16/FillP8/FillA8R8G8B8/FillR8G8B8/
FillA8/FillA8Additive. Validation + DecodedTexture wrapping stays ours.
X8R8G8B8, DXT1/3/5, SolidColor remain our implementations (no WB equiv).

Bonus: R5G6B5 + A4R4G4B4 formats now handled (previously fell to magenta).

9 conformance tests pass. Build 0 errors.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/AcDream.App/Rendering/TextureCache.cs

@@ -199,8 +199,9 @@ public sealed unsafe class TextureCache : IDisposable
 
         // Clipmap surfaces use palette indices 0..7 as transparent sentinels.
         bool isClipMap = surface.Type.HasFlag(SurfaceType.Base1ClipMap);
+        bool isAdditive = surface.Type.HasFlag(SurfaceType.Additive);
 
-        return SurfaceDecoder.DecodeRenderSurface(rs, effectivePalette, isClipMap);
+        return SurfaceDecoder.DecodeRenderSurface(rs, effectivePalette, isClipMap, isAdditive);
     }
 
     /// <summary>