This is the configuration surface through which abstract client mode is turned on for a given wiki. The working name is $wgWikiLambdaEnableAbstractClientMode, registered in extension.json alongside the three existing mode flags and defaulting to off so that loading the extension on any wiki does not implicitly enable the cross-wiki read path. The flag is checked in the same places the other mode flags are checked today: the special-page class's userCanExecute, the ResourceLoader module registration for any client-only front-end code, hook handlers that branch on mode, and the relevant REST/Action API entry points.

Acceptance criteria:

• $wgWikiLambdaEnableAbstractClientMode is registered in extension.json alongside the three existing mode flags, defaulting to off.
• The flag is checked at every site from the "checked in the same places the other mode flags are checked today" list (special-page userCanExecute, ResourceLoader module registration, hook handlers, REST/Action API entry points), verified by grep for each site.
• The four-combination matrix of supported production configurations (abstract.wikipedia.org, Wikifunctions, a local Wikipedia consuming Abstract Content, dev fallback) is documented and every combination has been exercised at least once in local development.
• Unsupported combinations (e.g. abstract on and abstract-client on simultaneously) produce a clear configuration error at extension load rather than undefined behaviour at first request.
• The flag's intended lifecycle (eventual merger into $wgWikiLambdaEnableClientMode) is recorded in the flag's extension.json description string, so that anyone discovering the flag in the config knows it is not permanent.