Add link facet + standard.site card refs to companion Bluesky post
The companion app.bsky.feed.post created on publish (Decision 0005)
was missing two things versus a reference standard.site post: the
article URL in the post text wasn't clickable, and the link card was
bare with no standard.site enrichment.
buildBskyPost now always emits an app.bsky.richtext.facet#link over
the URL's UTF-8 byte range (offsets computed with TextEncoder, so a
multibyte title shifts them correctly) and embeds associatedRefs —
the document and publication strongRefs, in that order — which the
Bluesky AppView resolves to render the rich card.
Embedding the document's strongRef means the post needs the document's
cid before it's written, while the document still keeps bskyPostRef
back at the post. To satisfy that mutual reference, publish is now
three writes: create document (no bskyPostRef) -> create post (facet +
associatedRefs) -> putRecord document to add bskyPostRef. Step 3
re-cids the document, leaving the post's document ref one version
stale; this is harmless because the AppView resolves associatedRefs by
URI, the same way standard.site itself behaves after an edit.
Publication gains a cid field (captured from list/create/putRecord
responses) threaded through PublishPanel as publicationCid. See
Decision 0013 for the full rationale; thumbnail enrichment is
deferred as a follow-up.