feat: orbit ghost/trail split, custom layers, Kepler period
ocaml-kepler:
- Analytic.period: orbital period from elements (seconds)
- Analytic.eccentricity: accessor
- Vec3: add sub, dot, cross
ocaml-globe/webgl/Orbit:
- add_ghost: persistent ghost orbits (survive clear_trails)
- clear_trails: clear only trails, keep ghosts
- clear_ghosts: clear only ghosts
- clear_lines: clear everything (backward compat)
- Draw order: ghosts first, then trails, then dots
ocaml-globe/webgl/Scene:
- layers.custom: list of draw functions called after orbits,
before coverage. For app-specific renderers (conjunction markers).
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