track: polish runtime sampling defaults

[Coasterpete]

Summary

Implements Milestone 151: Track Runtime Sampling Polish.

Changes

• Routes document-bound train-pose sampling through per-call CompiledTrackRuntime snapshots where safe.
• Centralizes internal sampling defaults through TrackSamplingOptions.Default.
• Refreshes historical transported-frame documentation to reflect current canonical runtime behavior.
• Adds parity/convergence coverage for transported-frame sample density.

Validation

• dotnet test QuantumCoasterWorks.sln --nologo
• 1721 tests passed.

Notes

This preserves public contracts and intentionally defers support spacing, matrix contract cleanup, frontend work, and debug sampler relocation.