Document reverse train junction routing

[ryanbarlow97]

Update the train guide for junction selection in reverse, leading-wheel route commitment, and points remaining locked until the whole train clears. Explain body orientation, saved routes during partial chunk loading, legacy data compatibility, and throttle tape conversion.

Validation: all 5 documentation checker tests pass; 692 local links and anchors across 180 Markdown pages and project navigation pass; the strict CoreProtect documentation build passes. Opus 5.5 high and Astra cleared this diff. Companion implementation: TF-Minecraft/VehicleFramework#57.

[ryanbarlow97]

Opus 5.5 high and Astra reviewed 54c0dd4c0948c9eb8485f2ef72d424fd6a321cec alongside TF-Minecraft/VehicleFramework#57. Both found no actionable documentation issues. The guide matches the implemented controls, route locking, partial-load persistence, orientation, and tape behavior.

Validation passed locally and in CI: 5 documentation checker tests, 692 links/anchors across 180 Markdown pages plus navigation, and the strict CoreProtect documentation build.