ConstA separate registered format, not a second extension on XLSX_FORMAT_KEY: a format's
handler is never told which of its extensions is being written, and macro-enabledness is exactly
such a per-extension decision. Both keys
share one reader and one writer; only the write side is opinionated (the same shape .sxl and
.json use).
Splitting the read side too is what makes a plain save round-trip: an origin remembers the
key it was read under, so an .xlsm opened as Excel would be saved back as .xlsx content
under an .xlsm name.
The registry key for macro-enabled Excel (
.xlsm).