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.
RecomputeTargetSet sampled TerrainSampler.HeightAt (a 169-point biome
blend kernel) 5x per column over the whole render distance on the main
thread every chunk crossing (~1,445 calls = ~250k biome lookups), causing
multi-frame hitches while moving. Memoize each chunk column's (min,max)
surface range, reused across ticks and pruned to the horizontal window, so
each column is sampled once. Exact (no terrain change); spike is amortized
to a thin edge strip per crossing and zero when stationary.
Pure-air chunks (above terrain) and chunks where every face culls
produce a zero-length mesh from ChunkMesher.Build(). Previously
InstallResult still allocated a real ChunkRenderer (VBO + VAO) and
registered a zero-vertex draw call. With RenderDistanceVertical=2 and
a player at surface Y, the topmost vertical layer is typically all
pure air -- roughly H*H = ~289 wasted renderers per spawn.
The chunk still gets added to _ready so SpawnReady, dirty-marking and
unload bookkeeping continue to work; DrawAll's iteration over the
renderer dictionary naturally skips entries that don't exist.
Adds Configuration.VerticalPriorityWeight (default 4). The priority
formula in ChunkStreamer.RecomputeTargetSet now uses
dx^2 + dz^2 + dy^2 * VerticalPriorityWeight, so horizontal chunks at
the player's Y are dispatched before equally-distant chunks above or
below. Fixes janky loading where deep-underground chunks were
competing with surface chunks for early dispatch slots.
TryGetDiff now returns an independent copy so callers can safely retain the
reference; removes the defensive copy from ChunkStreamer.DispatchLoad.
ChunkJob.ExistingChunk was never populated (DispatchRemesh uses closures
instead), so the field is removed along with its misleading record-level
comment.
