Add a native iOS markProcessed(shareId) method to the SharedImage plugin
that records an explicit completion signal in App Group UserDefaults
(sharedPhotoProcessedShareId) and logs "[ShareTarget] shareId=<id> marked
processed". The API is exposed to TypeScript via the plugin definitions and
a web no-op stub.
Deliberately does not delete files or clear share metadata (cleanup is
deferred to a later phase), is not yet called from application code, and
does not touch Android. Documents the share-target lifecycle and plugin API
in doc/share-target-ios-audit.md.
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