UpdateSpawnReady required every chunk in the spawn columns down to bedrock. With full-column loading, those deep chunks carry the lowest load priority in the whole render volume (dy^2 * VerticalPriorityWeight), so they stream in last -- the loading screen waited for nearly the entire render distance to generate (minutes on a tall mountain spawn, appearing as a hang).
Gate readiness on just the near-surface band (surfaceTop-2 .. surfaceTop) of the spawn area instead. Those are the highest-priority chunks, so spawn-ready now flips in ~350ms. Deep chunks are occluded at spawn and stream in afterwards. Extracts a SurfaceTopChunk helper shared with CollectTargets.
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