Introduce EphemerisAdapter interface for planetary positions

[jokiatgithub]

Context

The current heliocentric calculations should become one adapter behind a stable engine interface. That lets Starwatcher support approximate, fixture-backed, and future high-accuracy ephemeris sources.

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

Scope

• Define an EphemerisAdapter interface for body positions at a requested time.
• Move the current approximate planet calculations behind an ApproximatePlanetAdapter.
• Include adapter metadata such as source, supported bodies, valid date range, and accuracy notes.

Acceptance Criteria

• Current Mercury-through-Neptune positions can be generated through the adapter.
• Adapter output is independent from rendering code.
• Source and accuracy metadata are available to callers.
• Existing solar-system tests are preserved or migrated.