Implement GIS001 analyzer to detect inefficient Geometry.Union calls in loops

[Copilot]

This PR implements a new Roslyn analyzer (GIS001) that detects performance issues with NetTopologySuite's Geometry.Union method when called repeatedly inside loops.

Problem

Calling Geometry.Union() repeatedly in loops has poor performance characteristics:

// ❌ Poor performance - O(n²) complexity in many cases
Geometry result = null;
foreach (var geom in geometries)
{
    result = result.Union(geom);  // GIS001 warning here
}

This pattern is problematic because:

• Each union operation can be expensive
• Doing them individually creates quadratic complexity
• Performance degrades significantly with larger geometry collections

Solution

The analyzer detects this pattern and provides a code fix that replaces it with CascadedPolygonUnion.Union():

// ✅ Much better performance - optimized batch operations
Geometry result = CascadedPolygonUnion.Union(geometries);

Implementation Details

Analyzer Features

• GIS001: Detects Geometry.Union calls inside for, foreach, and while loops
• Scope: Only targets NetTopologySuite geometry types (checks namespace and inheritance)
• Precision: Ignores non-geometry Union calls (like LINQ Enumerable.Union)

Code Fix Provider

• Automatically replaces loop patterns with CascadedPolygonUnion.Union() calls
• Adds necessary using NetTopologySuite.Operation.Union; directive
• Preserves variable declarations and maintains code structure

Testing

• 6 comprehensive tests covering all loop types and edge cases
• MSTest framework following repository conventions
• CSharpCodeFixVerifier for analyzer and code fix validation
• All existing tests continue to pass (43/43)

Files Changed

• Optilizer/GeometryUnionInLoopAnalyzer.cs - Main analyzer implementation
• Optilizer.CodeFixes/GeometryUnionInLoopCodeFixProvider.cs - Code fix provider
• Optilizer.Test/GeometryUnionInLoopTests.cs - Comprehensive test suite
• README.md - Updated documentation with GIS001 details and examples

The implementation follows all repository conventions and integrates seamlessly with the existing Optilizer analyzer suite.

This pull request was created as a result of the following prompt from Copilot chat.

Create a new Roslyn analyzer (GIS001) that detects calls to NetTopologySuite's Geometry.Union inside a loop (for, foreach, while) and reports a warning due to performance concerns. Implement a code fixer to replace such code with NetTopologySuite.Operation.Union.CascadedPolygonUnion.Union or the most performant batch-union alternative. Add comprehensive analyzer and code fix tests. Follow repo conventions for structure and testing (MSTest & CSharpCodeFixVerifier).

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