Add preview viewer asset and style foundation

[Coasterpete]

Summary

• Add viewer-side asset loading infrastructure to Quantum.PreviewViewer.
• Add preview-styles.json style manifest support.
• Add /api/styles and /style-assets/... endpoints for safe local style asset loading.
• Add GLB/GLTF support for train style assets.
• Add train style selector UI and debug-box fallback behavior.
• Keep backend domain libraries and DebugViewportSnapshotV1 unchanged.

Architecture Notes

• Styles are viewer configuration, not snapshot data.
• Train asset replacement is role-based, starting with train.body.
• trackStyles is reserved for future rendering support.
• Debug boxes remain the fallback when assets are missing or fail to load.

Verification

• dotnet build QuantumCoasterWorks.sln --nologo
• dotnet test QuantumCoasterWorks.sln --nologo
• Passed: 1712/1712