const TILE_STRIDE: number
vec4 slots per tile: [face level i j], then per frame [c0] and [f0].
c0 is the cell the patch center falls in along each axis — an integer
carried as a float, exact because no rung's cells exceeds 2²⁴ on any body
in scope (Earth's one-meter rung is 6.4 × 10⁶) — and f0 is where in that
cell the center sits.