feat(O-T2): extract pure stateless helpers to AcDream.Core.Rendering.Wb

Verbatim copy of 5 WorldBuilder files into src/AcDream.Core/Rendering/Wb/:
- TextureHelpers.cs (pixel-format decoders, Chorizite Lib)
- SceneryHelpers.cs (scenery transforms, Chorizite Lib)
- TerrainUtils.cs, TerrainEntry.cs, CellSplitDirection.cs (WB.Shared Landscape)

Namespace migrated from WorldBuilder.* / Chorizite.OpenGLSDLBackend.Lib
to AcDream.Core.Rendering.Wb per O-D11. [MemoryPackable] stripped from
TerrainEntry per O-D10 (we don't serialize the struct).

Updated 3 source files + 1 test file to import from the new namespace.

Verbatim discipline (O-D1): only namespace + MemoryPack attribute changed.
All algorithm bodies byte-identical to upstream.

Note: TextureHelpers omits IsAlphaFormat() and GetCompressedLayerSize()
because those reference Chorizite.Core.Render.Enums.TextureFormat, a type
that has no path into AcDream.Core without adding an unwanted NuGet dep.
Neither method is called from Core or the test suite; the omission is safe.

Verified on main checkout: dotnet build green (0 errors), dotnet test
green — Failed: 8, Passed: 1147, Skipped: 0, Total: 1155 (baseline maintained).
TextureDecodeConformanceTests (9/9) pass byte-for-byte after namespace swap.
AcDream.Core project alone builds green in this worktree (App-layer failures
are pre-existing, blocked by empty WB submodule, addressed in Tasks 3+4).

Spec: docs/superpowers/specs/2026-05-21-phase-o-dat-path-unification-design.md

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

tests/AcDream.Core.Tests/Textures/TextureDecodeConformanceTests.cs

@@ -1,4 +1,4 @@
-using Chorizite.OpenGLSDLBackend.Lib;
+using AcDream.Core.Rendering.Wb;
 using DatReaderWriter.DBObjs;
 using DatReaderWriter.Types;