Add a first-class `checked: Bool?` to InstantPageListItem (orthogonal to the
ordered-list `num`), replacing the prior sentinel-string-in-num prototype.
- Data model: enum third associated value on .text/.blocks; Postbox + a new
FlatBuffers `checkState` tri-state field (0=nil,1=unchecked,2=checked,
backward-compatible); Equatable.
- Transmission: read/write the native API `checkbox`=flags.0 / `checked`=flags.1
bits on all four PageListItem / PageListOrderedItem constructors, so checkbox
state survives the server round-trip for sender and recipients.
- Markdown: forward parser routes detected `[ ]`/`[x]` into `checked` (keeping
the real number for ordered items); reverse converter emits `- [ ]`/`- [x]`
so editing a rich message round-trips.
- Display: V1 + V2 layout detect via `item.checked`; the V2 renderer draws real
CheckNode artwork (was a placeholder); preview text shows a checkbox glyph
instead of leaking the old sentinel.
Spec: docs/superpowers/specs/2026-05-27-instantpage-list-checkbox-design.md
Plan: docs/superpowers/plans/2026-05-27-instantpage-list-checkbox.md
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Reconstruct markdown source from a stored InstantPage to populate the edit
field, then re-classify on save (the inverse of the send path). Adds the
InstantPageToMarkdown converter, edit-field population and save-time
re-classification in ChatControllerLoadDisplayNode, and a shared InstantPage
previewText surfaced through MessageContentKind for reply/pinned/forward
previews. Documented in CLAUDE.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>