fix(pagination): split oversized tables whose only merges are horizontal

[karlei-irving]

Fixes #807.

The problem

trySplitSimpleOversizedTable rejects any table containing [colspan]:

table.querySelector("table, [rowspan], [colspan], [data-footnote-id]")

A rejected oversized table takes the whole-block overflow fallback: all of its rows land in a single page box whose content area clips, so everything past the first page-height is rendered but invisible. The shape this hurts most is a legal/business staple — a multi-page table whose section-header rows span all columns (w:gridSpan). On the document that surfaced this (landscape page, 25-row table, 7 colspan header rows, zero rowspan), 17 rows were silently hidden; Word paginates the same file to 6 pages.

Why the colspan reject is unnecessary

A colspan is horizontal merging, contained entirely within a single row — and this splitter only ever cuts between rows, so a row-boundary split can never break one. createSimpleTableFragment already clones the colgroup and whole rows (row.cloneNode(true)), so colspans travel intact with their cells, and every fragment is still re-measured with the existing bail-out to the old behavior if any fragment doesn't fit.

[rowspan] stays rejected: a vertical merge genuinely can cross a row-boundary split, and cloning rows cannot make that correct.

Changes

• npm/src/pagination.ts: drop [colspan] from the reject selector; update the fragment-builder doc comment to say which merges are unsafe (vertical) and why horizontal ones are fine.
• npm/tests/docxodus.spec.ts:
  • New test: an oversized table whose only merges are horizontal splits at row boundaries, all rows preserved in order, colspan cells intact (built by widening a row of the existing HW002-Table14.docx fixture to a full-width header row).
  • The existing merged-cells regression test keeps its fixture and all assertions — its table carries both rowspan and colspan, and the rowspan alone keeps the conservative fallback — retitled to "vertically merged cells" to say what it now pins.

Validation

• tsc --noEmit clean.
• Built pagination.bundle.js from this branch and drove it headless (Chrome) over the real document's converted HTML: 6 pages (matching Word), 5 table fragments, all 25 rows present in source order, all 7 colspan cells intact.
• We've been running the equivalent one-line change as a patch on the published 12.6.2 package in production use without regressions.