Refactor HeliocentricScene to consume normalized engine snapshots

[jokiatgithub]

Context

The renderer should become a consumer of engine output, not the place where engine assumptions leak. This prepares the scene for richer cameras, picking, and data-source switching.

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

Scope

• Update HeliocentricScene to accept a normalized scene snapshot.
• Keep the current visual behavior intact.
• Avoid mixing ephemeris calculations into rendering components.

Acceptance Criteria

• HeliocentricScene renders from a snapshot-like input.
• Current UI still renders all existing planets.
• Existing tests pass.
• Production build succeeds.