Define BodyState and SceneSnapshot engine types

[jokiatgithub]

Context

Rendering should consume a normalized engine snapshot instead of reaching directly into calculation details. This gives the project a clean boundary between astronomy and Three.js.

Roadmap: https://github.com/jokiatgithub/starwatcher/blob/main/docs/solar-system-engine-roadmap.md

Scope

• Add BodyState for calculated body position, velocity if available, display metadata, and source metadata.
• Add SceneSnapshot for timestamp, coordinate frame, bodies, scale profile, warnings, and provenance.
• Keep the types usable in tests without WebGL.

Acceptance Criteria

• Types are documented enough for future contributors.
• A snapshot can represent the current heliocentric scene.
• Snapshot metadata includes source/accuracy warnings.
• Existing tests stay green.