Root cause of the column-insert crash + "can't edit any cell". Since P5m,
`TableCell.children` holds WordsBlock[], but the slash-menu "Table" factory still
seeded cells with bare `emptyText()` (inline `text` nodes). `normalizeBlock` has
no case for `'text'` and returns undefined, so `normalizeTableCells` stored
`[undefined]` — which then crashed `getActiveMarksForSelection`'s walk and left
the cell with no real block to edit.
- built-ins.ts: wrap each seeded cell in a paragraph block (matches the demo doc,
the callout factory, and `createTableCell`).
- normalize.ts: `normalizeTableCells` now drops children that fail to normalize
and re-seeds an empty paragraph when none remain — self-heals any document that
already got a malformed table from the old factory.
Pairs with the defensive guard in selection-walkers (76b0d1c8). Engine tests
430/430 pass; changed files type-check clean.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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