* fix(shell): volume.balance no longer drains all volumes onto one server
The density-based capacity function reads per-disk VolumeInfos sizes, but
adjustAfterMove only updated VolumeCount and the selectedVolumes map. The
planner re-read a stale topology after every move, so the source node's
density never dropped and it kept moving volumes until that node was empty.
Move the volume's size accounting between disks after each planned move so the
density recomputes and the loop converges to an even distribution.
* refactor(shell): O(1) volume removal and direct disk lookup in adjustAfterMove
removeVolumeInfo swaps with the last element instead of shifting, and the disk
is fetched by key rather than ranging the DiskInfos map.
e332b97d52