[backnotprop]

SPIKE. Do not merge. This branch tries Pierre's @pierre/layouts engine for a free multi-pane layout beside the Inbox list (multi-thread view, option D).

What is here

• packages/vendor-pierre-layouts/: a pinned, verbatim copy of @pierre/layouts (Apache-2.0; the package is private and not on npm).
  • upstream/ holds their files byte for byte. src/ is our wrapper, and the app imports only that.
  • patches/ is empty. UPSTREAM.md records the source, the commit, the files left out and a sha256 manifest.
  • Also: NOTICE, and a README.md that says how to check against upstream, update, patch and retire it.
  • It is a private workspace package and is never published.
• scripts/check-vendored-pierre-layouts.ts checks the manifest. Its test fails on any edit inside upstream/, or on any import of upstream/ from outside the package.
• packages/inbox/spike/: the layout.
  • The main pane is App's real thread pane with its reply box, portalled in.
  • Other panes are read-only real threads, plus one file pane.
  • Row menu items "Open in a pane" and "Open as a tab", and dragging a row onto a pane.
  • Split, move, resize, tabs, zoom, close, and save/restore in localStorage.

Gate

The spike exists only in a build run with INBOX_LAYOUT_SPIKE=1:

• a Vite define, constant-folded, drops the lazy chunk and the engine from every other build;
• even then, it shows only at ?layout-spike and at 900 px wide or more.

Known

• tests/inbox-structure.test.ts fails: packages/inbox/App.tsx goes over its line ceiling by 11 lines. This is acceptable for a branch that never merges.
• Panes after the first are read-only. A real reply box in each pane needs the thread-slot extraction from App.