feat(write): cell styles for xlsx and ods

[vaceslav]

Part 1 of the styled/columnar export (spec docs/superpowers/specs/2026-10-04-styled-columnar-export-design.md §1). Closes #86. Part of #67.

• CellStyle (fill, font colour/bold/italic, number format, date format, horizontal alignment, wrap, thin border), CellColor, CellFont, CellBorder.
• NumberFormat / DateFormat parse a supported subset of Excel format codes; anything else is refused at Parse, never at write time. Number and date formats are separate, so a date always keeps a date format.
• writer.Style(style) → StyleId (stamped per writer; a foreign id is refused); Write(value, StyleId) overloads. Up to 4096 styles per file.
• xlsx: styles.xml built at the end; ods: common cell styles and data styles in styles.xml, written at the end — styles may appear mid-stream. Csv ignores styles.
• Each (style, value kind) is resolved once and cached: nothing allocated per styled cell (measured in all three formats).
• Reader fix: a custom xlsx number format with a backslash or [ inside a quoted literal was misread as a date.
• Verified with OpenXmlValidator, our own import (styled and unstyled rows read back the same), and LibreOffice (formats shown as stated; fills and bold survive xlsx→ods and ods→xlsx).
• Allocation-counting tests run in a non-parallel collection: Gen2 collections from parallel tests trim the array pool and skew the counter.