🧹 Refactor deeply nested logic in ShapeClipboard

[MarsLuay]

🎯 What: Extracted copySingleShapeRelationship and updateRelationshipTarget helper functions from copyShapeRelationships in src/ShapeClipboard.ts.
💡 Why: Reduces deep nesting and complexity in relationship copying logic, improving code maintainability and readability.
✅ Verification: Ran npm run typecheck and relevant unit tests in tests/ (pptx-multi-select-clipboard-contract.test.mjs, audit-pptx-clipboard-find.test.mjs, matrix-pptx-clipboard-find.test.mjs).
✨ Result: Improved maintainability of src/ShapeClipboard.ts while preserving all existing functionality.

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PR created automatically by Jules for task 8507467548741953128 started by @MarsLuay

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