🧹 Refactor deeply nested text replacement in PresentationEngine

[MarsLuay]

🎯 What: Extracted the deeply nested per-slide XML parsing and text replacement logic in PresentationEngine.replaceText into a dedicated replaceSlideText private helper method.

💡 Why: Reduces nesting depth in replaceText, improving readability, maintainability, and code health while preserving existing behavior.

✅ Verification: Verified with npm run typecheck, running specific find/replace tests with node --test, and running the full unit test suite via npm test (all 688 tests pass).

✨ Result: Improved maintainability of src/PresentationEngine.ts.

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PR created automatically by Jules for task 7782542818661401832 started by @MarsLuay

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