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.
Add a World that stores chunks in a Dictionary keyed by chunk
coordinate, with floor-division world<->chunk coordinate mapping
(correct for negative coords). The mesher takes an optional neighbor
sampler so faces at chunk borders are culled against adjacent chunks
instead of always meshing the shell. Render one ChunkRenderer per
chunk, placed via the model matrix. Reorganize the World layer into
Blocks/ and Chunk/ sub-namespaces.
