SD-1799 Bug: Table - lost vertical merge

Status: PASS

The three elements/attributes touched by this PR are all correctly handled per the spec:

w:vMerge — The getTableCellVMerge helper returns vMerge.attributes?.['w:val'] || 'continue' when w:val is absent, which matches the spec: the default value of w:vMerge with no w:val attribute is continue (ST_Merge, §17.18.57). The two valid values (restart, continue) are the only ones checked in the rowspan loop. All correct.

w:gridBefore — Read from w:trPr as w:val (ST_DecimalNumber). The spec confirms this is a child of w:trPr with a single w:val attribute (§17.4.15), and omitting it defaults to zero — which the code handles correctly with the value > 0 guard. All correct.

w:gridSpan — Read from w:tcPr as w:val (ST_DecimalNumber). The spec states omitting it means a span of 1 (§17.4.17), which the return ... value > 0 ? value : 1 fallback handles correctly. All correct.

The logical change — matching continuation cells by grid-column position using gridBefore + gridSpan rather than by raw tc index — is the right interpretation of the spec model. The spec treats all cells as snapping to a shared column grid, so row-relative cell indexing was the bug. The new implementation is spec-conformant.