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.
