The change feed answers `?shares=1` with every note the caller can see, each
saying how it is held, plus a `revoked` list of notes that left them. Granting
a share moves the note past the recipient's cursor; ending one, or the owner
deleting the note, leaves a revocation on the same cursor. A recipient at edit
may push the note's text, and nothing else. Protocol 6, feature `shares`;
opt-in, so the floor stays at 3.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
