Add JPL Horizons reference fixture tests

[jokiatgithub]

Context

The roadmap calls for explicit accuracy tiers. A first reference fixture gives the project a truth anchor and prevents silent drift in planetary calculations.

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

Scope

• Choose one representative date, preferably J2000 or another well-documented reference date.
• Add checked-in fixture data from JPL Horizons output.
• Compare approximate adapter output against the fixture with explicit tolerances.
• Document the fixture source and tolerance expectations.

Acceptance Criteria

• Fixture source and date are documented.
• Tests name the body/date/axis when tolerance checks fail.
• Tolerances are realistic for the current approximate engine.
• npm run test passes.