🧹 [code health improvement] Refactor inspectPowerPointPackage into modular helper functions

[MarsLuay]

🎯 What: Refactored the inspectPowerPointPackage function in src/PowerPointPackage.ts by decomposing it into two modular helper functions: readZipCentralDirectoryHeader and parseZipCentralDirectoryEntries.

💡 Why: Reduces function length and complexity, improving maintainability and readability without altering runtime behavior or error reporting.

✅ Verification: Verified with npm run typecheck, npm run lint, and full test execution npm test (all 684 unit tests passing).

✨ Result: Clean, modular, and readable ZIP central directory inspection code in PowerPointPackage.ts.

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PR created automatically by Jules for task 9565430039112181999 started by @MarsLuay

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