Android half of ios#53, spec §6.23. Each item is an ordinary story
envelope carrying a `set` naming the sequence, its position and its size.
🔴 One envelope per item rather than an array of attachments. The array
fails the single-`att` guard on any build predating §6.23 and renders the
whole set as a message in the conversation; separate envelopes degrade to
several stories in order, which every existing build already draws.
🔴 `setExpiry` takes the earliest stamp. Items are required to carry the
same one, but a sender can write different ones, and if the latest won
the difference would keep part of a sequence alive after the rest had
gone. A zero is skipped rather than read as "expires at the epoch",
which would kill the whole set.
⚠️ `groupedStories` renders what arrived: `n` is a claim, any item can
fail to arrive, and a missing one is indistinguishable from one never
sent. Stories with no set are keyed so two of them cannot collide.
⚠️ The set is persisted, or a relaunch scatters every sequence back into
loose stories.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
