fix(dwg): Prevent integer underflow in class count calculation

[marevol]

Summary

• Fixes integer underflow bug in DWG class parsing that caused memory explosion (80GB+) and infinite loops
• Adds underflow protection to both dwgreader18.cpp and dwgreader21.cpp
• Adds comprehensive test coverage for the fix

Changes Made

• src/intern/dwgreader18.cpp: Add bounds check before calculating endDataPos to prevent unsigned integer underflow when maxClassNum <= 499
• src/intern/dwgreader21.cpp: Same fix applied for consistency across DWG format readers
• tests/test_class_underflow.cpp: Unit tests verifying the underflow fix logic with various boundary values
• tests/test_dwg_classes.cpp: Integration tests for DWG class parsing including memory safety checks
• CMakeLists.txt: Register new test executables

Technical Details

The original calculation endDataPos = maxClassNum - 499 caused unsigned integer wraparound when maxClassNum <= 499. For example:

• maxClassNum = 236 (common in AC1032 files from DWG TrueView)
• Original: 236 - 499 = 4294967033 (unsigned wraparound)
• Fixed: (236 > 499) ? (236 - 499) : 0 = 0

Since classes 0-499 are reserved built-in DWG classes and custom classes start at 500, files with maxClassNum <= 499 have no custom classes to parse, making endDataPos = 0 the correct value.

Testing

• Unit tests verify correct calculation for boundary values (0, 499, 500, 1000)
• Integration tests verify no memory explosion occurs with problematic class counts
• All existing tests continue to pass

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