Add rich-text facets to a Bluesky post so URLs render as links, @handles render as mentions (and notify the user), and #tags render as hashtags.

Documented steps

1. Compose the raw post text first, e.g. '✨ example mentioning @atproto.com to share the URL https://en.wikipedia.org/wiki/CBOR.' Do not use Markdown/HTML markup - Bluesky strips none of that automatically, facets are the only supported decoration mechanism.
2. For each mention found in the text (e.g. '@atproto.com'), resolve the handle to a DID: GET $PDSHOST/xrpc/com.atproto.identity.resolveHandle?handle=atproto.com -> {"did": "did:plc:ewvi7nxzyoun6zhxrhs64oiz"}. If the request returns 400 (handle not found), skip that mention - it will simply render as plain text.
3. Compute the byte offsets of each substring you want to annotate (mention text including the leading '@', the full URL, or the '#tag' including '#') using the UTF-8 encoding of the text. Byte offsets are zero-indexed; byteStart is inclusive and byteEnd is exclusive (end - start equals the byte length of the substring).
4. Do NOT use native JS string indices (.slice()/.indexOf() on a UTF-16 string) or Python character indices directly - encode the text to UTF-8 bytes first and compute offsets against the byte array, or use an official SDK's RichText helper which does this conversion for you.
5. Build one facet object per annotation: {"index": {"byteStart": 23, "byteEnd": 35}, "features": [{"$type": "app.bsky.richtext.facet#mention", "did": "did:plc:..."}]}. Use app.bsky.richtext.facet#link with a 'uri' field for links, and app.bsky.richtext.facet#tag with a 'tag' field (no leading '#', maxLength 640) for hashtags.
6. Attach the array of facets to the post record's 'facets' field alongside 'text' and 'createdAt', then createRecord as usual (collection app.bsky.feed.post).
7. Ensure facets do not overlap; sort them by byteStart and drop any that overlap a previous one before submitting, since overlapping facets have undefined rendering behavior.