Fix BitString Length property implementation

[konard]

🐛 Problem

The BitString Length property setter had several critical issues:

1. NotImplementedException for length decrease: Setting the length to a smaller value threw NotImplementedException
2. Confusing logic: Lines 75-83 contained unclear array clearing and bit masking logic with "What is going on here?" comments
3. Incorrect border updates: Min/max positive word boundaries weren't updated when shrinking the bit string
4. Edge case handling: Setting length to 0 failed, and partial word masking was unreliable

🔧 Solution

Core Implementation Changes

Replaced the problematic setter logic with:

• ✅ Proper length expansion: Clear new words and mask off extra bits correctly
• ✅ Length shrinking support: Clear words beyond new length and update borders
• ✅ Correct bit masking: Handle partial words properly when changing length
• ✅ Edge case handling: Support setting length to 0 and cross-word boundary operations
• ✅ Border management: Update min/max positive word tracking when shrinking

Key Fixes

1. Removed NotImplementedException - Length can now be decreased
2. Clear confusing logic - Replaced unclear code with well-commented, logical operations
3. Proper masking - Bits beyond the new length are properly cleared
4. Border updates - TryShrinkBorders() is called when shrinking to maintain internal state consistency

🧪 Testing

Added comprehensive test coverage for all Length property scenarios:

• ✅ LengthIncreaseTest - Verify length expansion preserves existing bits
• ✅ LengthDecreaseTest - Verify length shrinking works correctly
• ✅ LengthSetToZeroTest - Edge case of setting length to 0
• ✅ LengthCrossWordBoundaryTest - Operations that cross 64-bit word boundaries
• ✅ LengthPartialWordMaskingTest - Verify proper bit masking within words
• ✅ LengthSameValueTest - No-op when setting to same value

All existing tests continue to pass (26/26 tests passing).

📝 Example Usage

var bitString = new BitString(64);
bitString[5] = true;
bitString[45] = true;

// This now works (previously threw NotImplementedException)
bitString.Length = 32;
Console.WriteLine(bitString[5]);  // True (preserved)
// bitString[45] is no longer accessible

// Setting to 0 also works
bitString.Length = 0;

🔗 Fixes

Fixes #21

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🤖 Generated with Claude Code