Add streaming ItemReader for legacy .xls files

[kebbaman]

Fixes #215

Adds a low-memory streaming reader for legacy binary .xls (BIFF8/HSSF) files - the
HSSF counterpart to the existing StreamingXlsxItemReader.

What this adds

• StreamingXlsItemReader<T> and a package-private XlsSheet in the streaming
  package, both built on the existing Sheet (Iterable<String[]>) abstraction and
  AbstractExcelItemReader, so all current configuration, mapping, blank-row handling
  and restart behaviour are inherited unchanged.

Approach

• Reads HSSF records in pull mode via RecordFactoryInputStream#nextRecord() - the
  HSSF analogue of the StAX loop in StreamingSheet - assembling rows one at a time,
  so memory stays bounded regardless of file size.
• The shared strings table is read once from the workbook globals and shared across
  sheets; each sheet is exposed as its own stream positioned at its BoundSheetRecord
  offset, mirroring how the xlsx reader gives each sheet an independent stream.
• Formulas return the cached value, consistent with the streaming xlsx reader.

Notes

• Additive only: no changes to existing readers, abstractions, or dependencies.
• ./mvnw -B package is green (compile + io.spring.javaformat checkstyle + tests).
• Commits are DCO signed-off.