feat(write): sheet layout — header style, frozen panes, auto-filter, merged cells

[vaceslav]

Part 2 of the styled/columnar export (spec docs/superpowers/specs/2026-10-04-styled-columnar-export-design.md §2). Closes #87. Part of #67.

• SheetOptions on BeginSheet: a header style, frozen rows and columns, an auto-filter on the header row through the last row written. Csv ignores it.
• writer.Merge(rows, columns): the next cell is the top-left of a merged range. Covered positions are skipped automatically and written by the writer, also when a row ends early. Overlaps, ranges past the columns, a merge no cell followed and ranges still open at the end of a sheet are refused.
• xlsx: sheetViews pane, autoFilter + _xlnm._FilterDatabase, mergeCells (≤ 65,536 per sheet); covered cells of a styled range carry its style, so Excel draws a merged range's border around the whole range.
• ods: freeze in settings.xml, filters as table:database-ranges, spans + covered cells; a run of empty cells is one repeated cell.
• A sheet without merges pays one null check per cell, nothing allocated.
• Verified with OpenXmlValidator, our import, and LibreOffice (filters, merges and merged fills survive conversion both ways; a laid-out data + legend workbook opens and reads back). Frozen panes are checked structurally only — headless LibreOffice keeps no view state.