feat(ocaml-globe): gradient orbit trails with ghost orbit + prediction
Orbit rendering now shows three visual layers:
1. Ghost orbit — full orbital path as a dim grey line
2. Trail — gradient from bright (satellite position) fading to
transparent in the past, with dimmer prediction ahead
3. Dot — bright glowing point at current position
Per-vertex progress attribute drives the alpha gradient in the
fragment shader. Demo updated to showcase the effect.
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