Reshard a rolling

Reshard a rolling #

Change the number of shards of a rolling, online. Splitting raises the shard count (finer partition ranges, e.g. 1 → 2); merging lowers it (e.g. 2 → 1).

The reshard builds a hidden target rolling while the source keeps serving every read and write. Existing documents are streamed into the target with their _id preserved, the write tail is replayed until the target converges, and a single metadata commit then swaps the target in atomically. Documents stay addressable under the same _id and _key throughout, including for writes issued while the reshard is building.