Fix indexing operators to validate negative indices

[konard]

Summary

• Fixed indexing operators in Point<T> and LinkAddress<T> classes to properly validate negative indices
• Added check for index < 0 in Point indexer to prevent invalid negative access
• Refactored LinkAddress indexer to use cleaner index != 0 validation
• Both indexers now correctly throw IndexOutOfRangeException for all invalid indices

Changes Made

• Point.cs:60: Changed if (index < Size) to if (index < 0 || index >= Size)
• LinkAddress.cs:45: Changed if (index == 0) to if (index != 0) for better readability

Test plan

• ✅ Created and ran comprehensive indexer validation tests
• ✅ Verified negative indices now properly throw IndexOutOfRangeException
• ✅ Confirmed valid indices still return expected values
• ✅ All existing tests continue to pass

Fixes #62

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