Open research children as Lift or Zoom cards inside the root document

[MaggieAppleton]

What changes

Opening a research child now replaces only the root's document panel. The root's Chat stays put, because a child no longer has a Chat of its own: there is one Chat per root document. The child is only a document, with no tabs, header or Chat. The Document, Decisions and Build tabs and presence belong to the root.

There are two motion styles, behind a dev toggle (localStorage["chopin:dev:child-motion"] = "lift" | "zoom", default Lift), so we can choose after using both:

• Lift: the whole root sheet, tab row included, dims slightly and recedes. The child is a card stacked on top with a small title bar, and the root's tab row peeks above it. Clicking that row ("Back to {root}") returns; clicking a tab returns and switches. Escape goes up one level.
• Zoom: the root tabs stay live. A stack bar under them shows Back and the path, and the child zooms in from the card. The stack bar stays visible on every tab, and its crumb takes you back to the open child.

Compact (phone) layouts use the stack bar.

Motion uses the Web Animations API from the clicked card's rectangle:

• clip-path and transform only
• the dim trails the grow
• reverses from the current frame
• closing lands back on the card
• open 600 ms, close 250 ms
• reduced motion is a crossfade

Opening from the sidebar grows from the panel's lower centre.

Also fixed: opening a child reconnected the root document's socket, which remounted the root editor and lost its scroll.

Stacked on #465 (merged). This is slice 1 of the nested-documents work.

Screenshots

Lift, child open

Lift open motion, frames 2–4

Zoom, child open

Zoom open motion, frames 2–4

Lift with the root expanded

Compact (390px)

Accessibility

• While a child is open, the covered root content is inert. Tab never reaches it, and the root's Chat stays reachable.
• Escape in the root's Chat or top bar stays with the root.
• Focus returns to the opener after every close path: Back, Escape, browser Back, a tab, or the crumb.

Testing

• New e2e/research-child-motion.e2e.ts samples real frames. It checks:
  • growth starts from the card
  • the dim trails the grow
  • Escape or Back mid-open reverses cleanly, with no settle waits
  • Back then Forward ends open
  • the close lands on the card
  • the sidebar fallback works
  • reduced motion is a crossfade only
• Rewrote research-child-surface and research-child-recovery for the new presentation, with keyboard sweep, focus and close-path coverage.
• Unit tests cover the pure geometry module.
• The full bun run e2e passed before the final restack, except two unrelated flakes that passed on rerun. After the restack, bun run types, the design-contract check and the unit tests pass.

Design-contract review

Renewed in "Renew design-contract review for Lift and Zoom" after a data-flow review, with the maintainer's approval:

• workspaceProps(…) in document-workspace-host.tsx passes only data and callbacks.
• The flagged workspace.tsx sites are unchanged reviewed classes.

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