Fix #2966: convert formula result to Date when numFmt is date-like

[senoff]

Bug

Tracked at exceljs#2966.

When a cell holds a formula whose cached result is a numeric Excel date
serial and the cell's numFmt is a date format (e.g. m/d/yyyy), the
reader is supposed to materialize cell.value.result as a JS Date.

It does in the common case (<v>46143</v> with no t attribute), but
not when an off-spec writer marks the formula cell with t=\"str\"
and still puts a numeric serial inside <v>. After PR exceljs#2956's guard
in excelToDate, the reader stopped throwing Invalid Date for that
shape, but the value was left as the raw string "46143.20833333333"

• still wrong for any consumer that expects a Date.

The streaming WorkbookReader had the same gap and never applied
date conversion to formula results at all.

Fix

lib/xlsx/xform/sheet/cell-xform.js (reconcile, Formula branch) and
lib/stream/xlsx/worksheet-reader.js (formula cell handling) both now:

1. Detect numFmt is date-like (utils.isDateFmt).
2. If result is a string but parses as a finite number that round-trips
   exactly, coerce to that number first.
3. Pass through utils.excelToDate, which already handles plain numbers
   and (post-Fix the return value from dateToExcel() when it's passed a non-numeric value. exceljs/exceljs#2956) returns non-numeric input unchanged.

Genuine display strings like \"27/08/2025 19:33:34\" cannot be
reliably parsed as an Excel serial, so they fall through unchanged - we
never fabricate an Invalid Date.

Tests

spec/integration/issues/issue-2966-formula-result-date-numfmt.spec.js
covers four shapes:

• numeric <v>, no t attribute (regression baseline)
• numeric <v> with t=\"str\" (the actual bug)
• non-numeric display string with t=\"str\" (must not become Invalid Date)
• streaming reader: numeric serial with t=\"str\" becomes a Date

Fixtures are built programmatically: a workbook is written with the
real m/d/yyyy style, then the <c> element is rewritten in-place via
JSZip so the malformed shapes can be exercised without checking binary
fixtures into the repo.

npm run test:unit, npm run test:integration, and
npm run test:end-to-end all pass.

Notes

Two unrelated multi-line function calls in the touched files were
collapsed to single-line form to satisfy the prettier+eslint hook
(prettier wants trailingComma: \"all\", eslint enforces
comma-dangle.functions: \"never\"). Behavior is unchanged.