feat: support columns in marimo-pair

[mscolnick]

Incoming transactions with set-config column updates only touched
cell.config.column metadata, leaving the MultiColumn tree unchanged
so boundary-anchor transactions never actually reorganized cells
across columns on the replica. Sort set-config changes last and
dispatch a new rebuildCellColumns action that regroups cells via
MultiColumn.fromIdsAndColumns using the reorder-cells flat order.

My smoke test:

  * Started with 2 cells "import marimo as mo" and `print(123)`
  1. Add a dataframe in the second column
  2. Add a chart above that as well
  3. Move the chart back to the first column
  4. Move the 123 cell to a 3rd column
  5. Add markdown headers above each column
``

[vercel]

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview, Comment	Apr 15, 2026 9:31pm

[Copilot]

Pull request overview

Adds proper column-layout synchronization for marimo-pair replicas by ensuring incoming set-config column metadata updates are reflected in the MultiColumn tree (so cells physically move across columns, not just update metadata).

Changes:

• Sort incoming transaction changes so set-config is applied after structural changes, then trigger a column-tree rebuild when column metadata changed.
• Add a rebuildCellColumns reducer action that reconstructs MultiColumn from the flat order + each cell’s config.column.
• Add targeted reducer and transaction-application tests covering boundary-anchor column conventions and ordering edge cases.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
frontend/src/core/cells/document-changes.ts	Sorts set-config last during transaction application and dispatches rebuildCellColumns when column metadata changes.
frontend/src/core/cells/cells.ts	Introduces rebuildCellColumns reducer action to rebuild the MultiColumn tree from config.column.
frontend/src/core/cells/tests/cells.test.ts	Adds reducer-level tests for rebuildCellColumns behavior.
frontend/src/core/cells/tests/apply-transaction.test.ts	Adds transaction-level tests verifying column rebuild behavior for boundary-anchor conventions and ordering variants.

[manzt]

very nice