feat(ocaml-globe): add raycasting for 3D picking
New module Globe.Raycast (pure math, no WebGL):
- ray_from_screen: unproject screen pixel → world-space ray
- intersect_sphere: ray-sphere intersection (Earth surface)
- intersect_point: ray proximity test for satellite dots
- pick_nearest: find closest point to ray from a list
- pick_earth: find where ray hits the globe surface
- invert_mat4: 4x4 matrix inverse for unprojection
15 new tests: sphere hit/miss/inside/tangent/offset, point hit/miss/
behind, pick nearest/empty, earth hit/miss, matrix inversion identity/
roundtrip, ray-from-screen center. 40 total globe tests passing.
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