Add native editable document columns

[MaggieAppleton]

Summary

• Add native, directly editable two-column sections to the restricted MDX dialect and shared Lexical/Yjs registry.
• Add /columns, keyboard exit, responsive stacking, and content-preserving Unwrap columns.
• Compose two-image galleries from existing image nodes and editable caption paragraphs; add URL and alt-text repair for existing images.

Preview

These captures show the real editor on the current main layout, with the same gallery side by side in a wide document and stacked in a narrow document.

Checks

• 4,230 Bun tests and 7 Chromium columns tests pass on the merged branch, including two-reader sync, reconnect, reload, image repair, and no horizontal overflow.
• bun run types, bun run ci, bun run fix, and the web build pass.

Current limits

This slice accepts exactly two top-level columns. Caption styling applies to the paragraph immediately after an image paragraph; the caption remains independent prose. Planner operations do not yet address a nested column separately. The concurrent #415 guidance is unchanged.

[MaggieAppleton]

@copilot remove committed plans here and remove any tests that just prove the implementation vs. guarding against future regressions

[Copilot]

@copilot remove committed plans here and remove any tests that just prove the implementation vs. guarding against future regressions

Removed the committed implementation plan and the duplicate slash-command unit test; the browser test still covers insertion and caret placement. Commit: 4397c1c.