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>
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