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.
