words/engine: guard the mark-walk against undefined block slots

`getActiveMarksForSelection` → `collectBlockText` switched on `block.type`
without a null check. When a table cell (inside a column) momentarily held an
undefined child slot, the walk threw `Cannot read properties of undefined
(reading 'type')`. The throw propagated out of `insertBlockInColumn` →
`applyHistoryCommand` uncaught, aborting the command and leaving the editor
unable to edit anything (and the column "add block" silently failing).

Add an `if (!block) return` guard at the top of `collectBlockText` so a partial
slot is skipped instead of crashing the whole command. Surfaced via the Chrome
console (TypeError in selection-walkers.ts ← insert-block-types.ts:309).

Engine tests 42/42 pass; selection-walkers type-checks clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/uix/soma/components/words/engine/operations/selection-walkers.ts

@@ -67,10 +67,15 @@ export function getTextEntries(doc: WordsDocument): readonly WordsTextEntry[] {
 }
 
 function collectBlockText(
-	block: WordsDocument['children'][number],
+	block: WordsDocument['children'][number] | undefined,
 	path: WordsPath,
 	out: WordsTextEntry[]
 ): void {
+	// Robustness: a cell / column / callout child slot can momentarily be
+	// undefined during an editing transition. Skip it rather than throwing
+	// from the mark-walk — an uncaught error here aborts the command and
+	// leaves the editor unable to edit at all.
+	if (!block) return;
 	switch (block.type) {
 		case 'paragraph':
 		case 'heading':