* fix ec.balance failing to rebalance when all nodes share all volumes (#8793)
Two bugs in doBalanceEcRack prevented rebalancing:
1. Sorting by freeEcSlot instead of actual shard count caused incorrect
empty/full node selection when nodes have different total capacities.
2. The volume-level check skipped any volume already present on the
target node. When every node has a shard of every volume (common
with many EC volumes across N nodes with N shards each), no moves
were possible.
Fix: sort by actual shard count, and use a two-pass approach - first
prefer moving shards of volumes not on the target (best diversity),
then fall back to moving specific shard IDs not yet on the target.
* add test simulating real cluster topology from issue #8793
Uses the actual node addresses and mixed max capacities (80 vs 33)
from the reporter's 14-node cluster to verify ec.balance correctly
rebalances with heterogeneous node sizes.
* fix pass comments to match 0-indexed loop variable
Chris Lu
f256002d0b