Add rebalance tool with --dry-run support

Pure plan_rebalance() computes diffs between current and desired placement.
Execution copies blobs to new volumes, updates index, deletes from old.
Skips keys where copy fails. Includes unit tests for planning logic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib.rs

@@ -4,6 +4,7 @@ pub mod error;
 pub mod hasher;
 pub mod health;
 pub mod server;
+pub mod rebalance;
 pub mod rebuild;
 pub mod volume;