feat(document): style a cell of an ods file

[andiwand]

Step 2 of the cell-formatting plan (#934).

Change

• Sheet::set_cell_style(column, row, TableCellStyle, TextStyle) and the setCellStyle op:

  {"op": "setCellStyle", "sheet": 0, "column": 1, "row": 2,
   "style": {"fill": "#ffff00", "bold": true, "align": "center"}}

  The keys are those of setTextStyle, plus fill (#rrggbb or null) and align (left, center, right). highlight refuses, and so does any style field that no engine writes.

• The ods writer claims the cell as a value write does, and points table:style-name at a fresh automatic ce<n> style. That style copies the automatic style that the cell shows (its own, else the row or column default) with the delta, or it inherits from a named style. One base and one delta make one style, so formatting a range adds one style and not one per cell.

• The fill is fo:background-color, the alignment is fo:text-align with style:text-align-source="fix", and the text keys are written as setTextStyle writes them.

• Other engines refuse, because the hook's default throws. xlsx comes in the next PR.

Two fixes found on the way

• Covered position: a value written into a covered cell was written into the sheet outside the merge. The index holds no covered cell, so is_covered now walks the row in the DOM, and both writes refuse.
• Empty cell after a merge: claim_cell took the start of a run from the end of the cell before it. After a span that is wrong, so a write into an empty cell right of a merge threw. It now takes the start from the run itself. The regression test fails without the fix.

Proof

• 13 new OdfSheetStyle tests on inline flat sheets: a shared style is copied, a named one is inherited from, a repeated run is cut, a cell past the sheet is made, one delta makes one style, a fill taken away is transparent, alignment, a formula cell, refusals and the JSON op.
• The full suite passes (1883 tests). The reference output does not change.
• Senza nome 1.ods styled with seven ops and saved: LibreOffice loads it and shows every op as written: fills, bold, italic, underline, colour, strikethrough at 16pt, a fill taken away, and a cell grown past the table. Our own render of the saved file matches.

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