feat(ocaml-globe): additive blending coverage that lights up the globe
Coverage footprints now use filled triangles with additive blending
(GL_SRC_ALPHA, GL_ONE) instead of outline lines. This brightens
whatever is underneath — dots, grid, orbits all glow within the
satellite's field of view. Radial falloff from bright center to
transparent edge gives a spotlight effect.
New: Globe.Geometry.filled_circle_on_sphere for triangle generation.