fix(html): stop a cell spill at a covered cell, and advance the output pins

[andiwand]

main fails the reference comparison, because the references were behind. While I checked the differences, I found one regression, and this PR fixes it.

Fix. The cell index of an ods holds no covered cell, so a lookup at a covered position gives a blank cell. Since #1008, the spill scan therefore let text run over the merged cell from a row above, for example (KWH) over KWH Meter in style-various-2.ods. The sheet loop now records, per column, the row where a span from above ends, and the scan stops at a covered column, as LibreOffice does. A new test covers it, and the test fails without the fix.

References. Pinned to public 78e3081 and private 2e4e8ef, regenerated with this fix. I checked each kind of change against the source files:

• Blank cells lose their clip or overflow (fix: bound spreadsheet width and skip empty spill scans #1095). No pixel changes.
• A cell style that states transparent no longer inherits a colour (fix: clear inherited ODF backgrounds and borders #1089): Senza nome 1.ods, efficiency-big-1.ods.
• A spill stops at a covered column (this fix). Every changed cell that I checked stops at a merged cell.
• xlsx: cached values keep full precision, and a value written with an exponent (1E-6) is read. The shown text does not change.
• PDF: the text is the same on every page, except that the cover title of "Classical Music … Intermediate" now appears (fix(pdf): parse CMap tokens and validate CID records #1122). After a 1.5 px blur, only small icon colours on the mova pages differ (fix(pdf): preserve shading function stream references #1116).
• Scripts: document.js, editing.js, sheet-editing.js, spreadsheet.js and text.js (fix: address post-v7.4.0 spreadsheet review findings #1018, fix: preserve Unicode pairs during browser deletion #1094).

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