## Summary
Tag files with `.xls` extension were accepted by the extension check in
`parseTagSourceByFilename` but always produced an empty tag index:
`parseXLSXTagSource` used only `excelize.OpenReader`, which rejects OLE
BIFF workbooks (`unsupported workbook file format`). This makes legacy
Excel 97-2003 tag files unusable while the docs claim `XLSX, XLS, CSV,
TXT` support.
This PR routes OLE-magic `.xls` bytes to a new BIFF8 reader
(`go-excel-reader`, already used by the document `XLSParser`), applying
the identical per-row rule as `.xlsx`.
## Related Issue
None (follow-up of #10602 / #10660 which fixed document-side `.xls`; tag
side was left behind).
## Changes
- `internal/ingestion/component/tag_source.go`
- Detect OLE compound-document magic and route to new
`parseBIFF8TagSource` (fallback to `excelize` for OOXML-renamed `.xls`).
- `parseBIFF8TagSource`: per-sheet, no-header, first two non-empty cells
form `(content, tags)`; single-cell rows accumulate; extra cells
ignored.
- Extract shared `accumulateTagCells` helper (removes xlsx/BIFF8 loop
duplication); simplify `tagCellString` (drop redundant `string` case).
- Fix `unsupported extension` error text to include `.xls`.
- `internal/ingestion/component/tag_biff8_test.go` (new):
`TestParseBIFF8TagSource` with an xlwt-generated BIFF8 fixture (3
examples incl. single-cell accumulation).
## Test Plan
- `bash build.sh --test ./internal/ingestion/component/ -run
'TestParseBIFF8TagSource|TestParseTagSourceByFilename$|TestParseXLSXTagSource'
-v` — all PASS.
- `bash build.sh --test ./internal/ingestion/component/` — full package
PASS (~16s).
- Real-file check: document `XLSParser` parses 4 genuine `.xls` fixtures
(`produced=true`); same bytes via tag path previously returned
`count=0`, now return 3 tag examples.
jay77721
3e313daf6f