Milestone 149 preview viewer spike

[Coasterpete]

Summary

• Add Quantum.PreviewViewer, a separate ASP.NET Core + Three.js spike viewer for DebugViewportSnapshotV1.
• Render centerlines, frame axes, diagnostics, and train/debug boxes.
• Add play/pause, distance scrubber, speed control, orbit/pan/zoom, follow camera, PNG capture, and WebM recording.
• Add snapshot catalog/path tests and README run notes.

Notes

• This is a prototype viewer, not a production editor.
• Backend domain libraries remain unchanged.
• Three.js is loaded from a pinned CDN for this spike.

Verification

• dotnet test QuantumCoasterWorks.sln --nologo
• dotnet build QuantumCoasterWorks.sln --nologo
• Passed: 1708/1708