office-stamper: Configurable Excel Merge Strategies

Excel as a data source was already in office-stamper (I wrote about it last year). What this commit adds is that “merge the Excel data into the context” is not one operation; it’s a choice.

When your workbook has several sheets, you can treat them two ways:

• MAP — each sheet becomes a named entry in the context, keyed by sheet name. This is the default.
• JOIN — the sheets are related rows sharing a key, joined into one table before stamping.

ExcelStrategy turns that choice into a CLI flag group:

officestamper -t template.docx -d data.xlsx \
    --excel-merge-strategy JOIN --excel-join-key ID

--excel-join-key defaults to ID and only matters for JOIN.

The previous behaviour assumed MAP, so anyone whose workbook looked like a relational join had to pre-join it by hand. Making the strategy explicit moves that knowledge into the tool.