Add read cache for zip archive entries in XLSX reader

[kemo]

Summary

• Adds an in-memory cache to getFromZipArchive() in the XLSX reader to avoid re-reading the same zip entries
• The method is called 17+ times during parsing with up to 3 fallback getFromName() attempts each — files like styles.xml and theme.xml may be decompressed multiple times
• Cache is cleared at the start of each entry point (canRead, listWorksheetNames, listWorksheetInfo, loadSpreadsheetFromFile)

Changes

• src/PhpSpreadsheet/Reader/Xlsx.php: Added $zipCache array, cache check in getFromZipArchive(), clearZipCache() calls at entry points
• tests/PhpSpreadsheetTests/Reader/Xlsx/ZipReadCacheTest.php: 5 tests covering correctness, repeated loads, different files, list operations
• tests/PhpSpreadsheetTests/Benchmark/XlsxZipReadCacheBenchmark.php: Benchmark with complex multi-sheet XLSX

Test plan

• Verify XLSX files load correctly with the cache
• Verify cache is properly cleared between separate load operations
• Verify listWorksheetNames() and listWorksheetInfo() work correctly
• Run benchmark: vendor/bin/phpunit --group benchmark --filter XlsxZipReadCacheBenchmark --stderr

[kemo]

Benchmark Results (xlsx-zip-read-cache)

Targeted benchmark: read XLSX files with varying zip archive complexity (single and multi-sheet).

Benchmark	Master	Branch	Delta
issue.3654c (1.7MB)	353ms	360ms	+2%
issue.4248 (696K)	53ms	52ms	~same
issue.3654 (558K)	38ms	38ms	~same
issue.2362 (253K)	1,035ms	1,061ms	+3%
27template (243K)	4.7ms	4.5ms	~same
Generated 10 sheets (0.3MB)	2,162ms	2,124ms	-2%
Generated 25 sheets (0.7MB)	5,525ms	5,533ms	~same

No meaningful difference. The zip read cache doesn't help when each archive entry is read only once during a load. The benefit would show in scenarios where the same zip entries are accessed multiple times (e.g. repeated partial reads or multi-pass processing).

[oleibman]

Thank you for the summaries. For purposes of prioritization, I am going to put the PRs with less demonstrable improvement, including this one, in draft status for now. This does not mean that I will not return to them.