Extract shared saveArticleToStash helper (dedupe 5 copies)

The build-a-StashClient-and-branch-on-the-result dance was
independently copy-pasted at 5 call sites (ShareReceiverActivity,
ReaderScreen x2, LinkReaderScreen x2). StashSaveOutcome carries the
exact snackbar text plus the artifact id, as a sealed class so a
caller that cares about the Saved/AlreadySaved distinction can still
pattern-match instead of parsing the message back apart.

Not yet adopted at any call site -- follow-up commits switch each one
over.

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