Add .xls (Excel Binary File Format / BIFF8) read+write support

[Mearman]

Add read+write support for .xls, the Excel Binary File Format (BIFF8), as defined by [MS-XLS]. The workbook's BIFF8 record stream lives inside a Workbook stream in an [MS-CFB] compound file — archive-codec's existing CFB reader already gets a codec to the raw stream bytes; parsing the BIFF8 record structure itself (and mapping it to/from the shared document-schema.js content schema, matching how ooxml.js's xlsx support and odf.js's ods support both target the same schema) is the actual work.

Part of #85. This is the one format the original out-of-scope assessment flagged as worth considering on its own (read-only), since BIFF8 is comparatively more tractable than the Word/PowerPoint binary formats — but full read+write is now the target alongside .doc and .ppt.

[Mearman]

Opened #844 with a read-only xls-codec package. Leaving this issue open — it asks for read+write, and this is the read half.

What shipped. The BIFF8 record framing, Continue handling, all three string shapes, RkNumber, the error vocabulary, substream splitting, the workbook globals (BoundSheet8/SST/Format/XF/Date1904), the worksheet grid (Dimensions/Row/ColInfo/MergeCells), the whole cell-value family (Blank, MulBlank, RK, MulRk, Number, BoolErr, LabelSst, Label, Formula+String), number-format classification, date serials, and the mapping onto ContentDocument — the same shape readXlsxContent produces. 190 unit tests, plus smoke and workerd suites.

What's still open, in rough order of value:

1. The write path. The main remaining item. Needs a compound-file writer (archive-codec reads but does not write CFB), a BOF history block, the XF/Font/Format table with the fifteen mandatory style records preceding any cell format, and an Index/DBCell row-block structure whose file offsets have to agree with where the records actually land. I deliberately didn't attempt a partial version — a writer that emits files Excel rejects is worse than no writer.
2. Formula expressions. Cached results are read, so cells show the right values, but ContentSheetCell.formula stays absent. BIFF8 stores the expression as a compiled Ptg token stream, not text; recovering it means implementing the Ptg vocabulary plus shared-formula (ShrFmla) and external-reference (SupBook/ExternSheet) resolution. This is the largest single piece of remaining read work and probably deserves its own issue.
3. Cell decoration — fill, borders, alignment from CellXF. Needs the Palette record plus the default 56-colour table to resolve colour indices.
4. Print settings — currently Excel's documented "Normal" preset rather than the file's own. Needs Setup (including its paper-size code table), the four margin records, PrintGrid, and PrintRowCol.
5. Workbook metadata — title/author/dates live in the \x05SummaryInformation property-set stream ([MS-OSHARED]), a separate format from BIFF8 entirely. metadata is currently {}.
6. Not read at all: charts, drawings/images, cell comments (Note/Txo), data validation, conditional formatting, defined names (Lbl).
7. Registry wiring into documents.js's conversion engine — the codec currently stands alone.
8. Encrypted workbooks (FilePass) are detected and refused rather than decrypted; probably should stay that way, but noting it.

One thing worth recording while it's fresh: the number-format classifier in xls-codec duplicates ooxml.js's typed/xlsx/number-format.ts. They solve the identical problem — the format mini-language is the same one, since OOXML inherited it from BIFF — and the right fix is extracting both into a foundation module rather than having a format codec depend on a sibling format codec. Not done here to keep the PR scoped, but it's real duplication and will drift if left.

Also incidental: .xlr (Microsoft Works 9 spreadsheets, from the since-closed #818) reads through this codec for free, since it's BIFF8 in the same container with an extra Works-specific stream the reader ignores. There's a test pinning it.

[Mearman]

xls-codec now has a real BIFF8 write path (writeXlsContent/writeXls, PR #879): a spec-grounded Workbook stream wrapped in a real [MS-CFB] compound file via archive-codec's writeCompoundFile.

Covered, verified by round trip against this package's own reader: every ContentCellValue kind BIFF8 can hold (number, percentage, currency, boolean, date, time, dateTime, string, error), merged ranges, row heights/hidden rows, column widths/hidden columns, multiple sheets, explicit/default number formats deduplicated across the workbook via the shared excel-number-format classifier, a deduplicated shared-string table, and genuine BOF/Font/Format/XF/Style/BoundSheet8/Dimensions/ColInfo/Row/Number/BoolErr/LabelSst/MergeCells/EOF records.

Not covered: formula expressions (the reader doesn't recover them either), cell decoration (fill/borders/alignment/per-cell font -- the reader doesn't read it back either), images/embedded objects/comments/data validation/conditional formatting/defined names, print settings and workbook metadata, UI/interop bookkeeping records, and Continue-record splitting for oversized records.

Not wired into documents.js's conversion registry yet. Leaving this open, since it's still short of the read+write parity the issue asks for.