Optimize XLS (BIFF8) reader performance with reduced per-cell overhead

[kemo]

Summary

• Eliminates redundant updateInCollection() calls during cell reads by introducing Cell::setXfIndexNoUpdate() and reordering XF index assignment before setValueExplicit() across all 9 cell reader methods (readRk, readLabelSst, readNumber, readFormula, readBoolErr, readLabel, readMulRk, readMulBlank, readBlank)
• Replaces O(n²) char-by-char string concatenation in SST reader with O(n) str_split/implode for both compressed-to-uncompressed expansion and CONTINUE record splicing
• Caches read filter reference and sheet title per-sheet to avoid repeated method calls on every cell
• Pre-computes cell coordinate strings ($columnString . $rowIndex) in all cell reader methods instead of inline concatenation
• Caches SST entry locally in readLabelSst and formula result type byte in readFormula to reduce repeated lookups

Changes

• src/PhpSpreadsheet/Cell/Cell.php: New setXfIndexNoUpdate() internal method that sets XF index without triggering updateInCollection()
• src/PhpSpreadsheet/Reader/Xls.php: All 9 cell reader methods optimized, SST string concatenation replaced, cached $phpSheetTitle and $cachedReadFilter properties added, splice offset loops converted to indexed for-loops
• src/PhpSpreadsheet/Reader/Xls/LoadSpreadsheet.php: Sets cached read filter and sheet title at the start of each sheet iteration

Benchmark results (best of 3 runs, 5 iterations each)

Benchmark	Master	Optimized	Change
Real XLS fixtures (6 files)	119.12 ms	114.64 ms	-3.8%
Synthetic (150k cells, 5 files)	6,606 ms	6,461 ms	-2.2%
SST-heavy (100k string cells)	4,197 ms	3,977 ms	-5.3%

Test plan

• Verify all 82 XLS reader tests pass (1425 assertions)
• Verify cell values, styles, and XF indices are identical to master
• Verify rich text formatting in SST strings is preserved
• Verify shared formulas resolve correctly
• Verify blank cells retain their style information

[oleibman]

Is there a Benchmark test you can add to this PR?

[kemo]

Sure — I'll add a benchmark test for the BIFF8 reader to this PR.