CSV write must make multi-sheet selection explicit, not silently drop sheets

Cluster: csv

Scenario

A user builds a workbook with several worksheets and asks the library to serialize it to CSV. CSV is a flat, single-table format with no concept of multiple sheets, so the writer can represent only one. Today it silently picks the first worksheet and drops the rest, with no error, no warning, and no way to influence the choice. Users are surprised to find only one sheet's data in the output and cannot target a specific sheet. The recurring confusion over many years shows the silent drop, not the single-sheet limitation itself, is the actual pain.

Spec note, not a corpus case: the current output is well-formed, containing the wrong sheet for a multi-sheet workbook, so there is no malformed serialization to assert. The defect is a silent-data-loss usability choice plus an undecided default. The durable value is the selection contract.

Desired behavior

• CSV write accepts an option naming the worksheet to serialize, by name or index. When provided, that sheet is written; if the named or indexed sheet does not exist, fail with a clear error naming the available sheets rather than silently falling back.
• When the option is omitted and the workbook has exactly one worksheet, write it with no ceremony, which is the current happy path.
• When the option is omitted and the workbook has more than one worksheet, surface the ambiguity rather than silently taking the first. Erroring is safer, since it admits no silent data loss, and fits the fork's correctness-first stance, though it is a harder break for callers relying on first-sheet behavior. The alternative is default-to-first with a diagnostic.

Open questions

• Omitted selection with multiple sheets: error, or warn-and-default? Correctness against convenience.
• A batch mode that writes one CSV per worksheet (multiple named buffers, or a directory) for users who genuinely want every sheet exported?
• How is "active sheet" resolved against "first sheet" when both concepts exist?
• The selection option lives under the existing CSV write options object.

Related: csv-write-date-format-honored, path-reader-is-node-only-clear-error.