[Drift] New `--incremental-only` compact flag added; MongoDB V3 storage compaction restructured (powersync-service #752)

[benitav]

What's changing: PowerSync Service restructures how MongoDB storage's experimental V3 format compacts bucket data: initial replication now triggers a cheaper "chunk-merge compact" instead of checksum pre-calculation, compact runs only check buckets that may need it (so they can run on much shorter intervals), and multiple compact jobs can run concurrently. This does not affect MongoDB V1 storage, Postgres storage, or parameter-index compacting. The V3 bucket-state collection's fields/indexes changed (a breaking change for V3 storage, with no migration path, but V3 storage remains explicitly unstable). The compact command also gains a new --incremental-only option, intended for jobs running on a short interval, which skips streams using V1 storage.

High priority — existing docs are now inaccurate or incomplete
None identified. MongoDB V3 storage remains experimental/unstable and is not documented anywhere in the docs repo, so nothing existing is contradicted.

For reviewer consideration — worth documenting

• New --incremental-only flag on the compact command. Compacting Buckets (maintenance-ops/compacting-buckets.mdx → https://docs.powersync.com/maintenance-ops/compacting-buckets), "Self-Hosted PowerSync" section, currently only says "the compact command in the Docker image can be used to compact all buckets" without documenting any flags — so nothing is wrong, but self-hosted operators running compact on a schedule have no way to discover this new option from docs.
• MongoDB V3 storage's compaction behavior changed substantially (chunk-merge compacting, concurrent compact jobs, bucket-state schema). No docs page currently mentions V3 storage at all, consistent with the already-open V3 object-storage item in [Drift] Object storage (S3) support added for MongoDB bucket storage V3, with configurable region and path style (powersync-service #728) #547 — reviewer's call on whether/when V3 storage as a whole warrants documentation given its unstable status.

Source PR: powersync-ja/powersync-service#752. Merged 2026-08-18.

Filed automatically by Claude Code (claude-sonnet-5). A human must verify the scope, prepare the docs PR, and publish it only once this update has been released.