Document OPFSWriteAheadVFS for the JavaScript Web SDK

[mintlify]

Summary

Documents the new WASQLiteVFS.OPFSWriteAheadVFS option added to @powersync/web (minor release). This is the only VFS that supports concurrent reads alongside writes by hosting one or more read-only SQLite connections in additional dedicated workers.

Trigger

This PR was triggered by powersync-ja/powersync-js#937 ("Shared worker: Immediate reconnect on connection interrupt"), which shipped alongside several other changes. The user-facing addition that needed documentation is the new OPFSWriteAheadVFS.

Changes

• Added an OPFSWriteAheadVFS subsection under "SQLite Virtual File Systems" in client-sdks/reference/javascript-web.mdx, covering when to use it, the new additionalReaders option, browser/web worker requirements, and a configuration example.
• Updated the VFS compatibility matrix with a new "Concurrent Reads" column and a row for OPFSWriteAheadVFS.

Other changes in the source PR (no doc update needed)

• Immediate reconnect on connection interrupt when using the Rust sync client on web — internal/transparent improvement.
• Removed deprecated unload listener — internal cleanup.
• Removed experimental label from raw tables in the SDK code — current docs do not call raw tables experimental, so no copy change required.
• Updated @journeyapps/wa-sqlite and PowerSync SQLite core extension — internal version bumps.
• Diagnostics app deeplink endpoint query parameter and Capacitor iOS WebSocket fix — out of scope for the SDK reference.

cc @benitav @simolus3 for review.

[simolus3]

Duplicate of #400.