Milestone 147: lock layout package v2 shape

[Coasterpete]

Summary

Locks the intended TrackLayoutPackageV2 JSON shape before implementation.

This is a docs-only contract-shape milestone. It does not add V2 DTOs, schema, validators, mappers, or runtime behavior.

Shape Decisions

• Contract remains quantum.track_layout_package.
• Version is 2.
• Root fields are contract, version, metadata, startPose, sections, banking, and heartline.
• metadata.layoutId is optional and nullable.
• V2 carries forward V1-compatible start pose, section, and banking shapes.
• heartline is required by the intended schema but nullable.
• First non-null heartline kind is constantOffset.
• Heartline offsets use centerlineStation and sampled frame axes.
• V1-to-V2 migration sets heartline: null.

Explicitly Deferred

• V2 DTOs
• V2 schema
• V2 validator
• V2 mapper
• Runtime behavior
• Force profile package
• Train consist package
• Block/operations package

Validation

• dotnet test QuantumCoasterWorks.sln --no-restore
• git diff --check

Tests: 1,668 / 1,668 passed.