feat: per-arc endpoint elevation

[TomPlanche]

Add optional fromElevation and toElevation properties to the Arc interface, mirroring the existing per-marker elevation pattern (see #113).

When set, the arc endpoint uses GLOBE_R + fromElevation / toElevation instead of the global markerElevation uniform. When omitted, behavior is unchanged.

Usage

arcs: [{
    from: [lat1, lon1],
    to: [lat2, lon2],
    fromElevation: 0, // matches a marker with elevation: 0
    toElevation: 0,
}]

Omitting either property falls back to the global markerElevation.

[vercel]

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