Add VSMEF003 analyzer to detect export type mismatches

[Copilot]

This PR implements a new Roslyn analyzer VSMEF003 that detects when MEF [Export(typeof(T))] attributes specify a type that is not implemented by the exporting class.

Problem

Previously, MEF parts could declare exports for types they don't actually implement, leading to potential runtime composition failures:

interface ICalculator
{
    int Add(int a, int b);
}

[Export(typeof(ICalculator))]  // ❌ Should warn - TextProcessor doesn't implement ICalculator
public class TextProcessor
{
    public string ProcessText(string input) => input.ToUpper();
}

Solution

The new analyzer warns when:

• A class exports an interface it doesn't implement
• A class exports a base class it doesn't inherit from
• A class exports an unrelated type

The analyzer correctly handles:

• Both MEF v1 (System.ComponentModel.Composition) and MEF v2 (System.Composition) attributes
• Interface implementation checking via AllInterfaces
• Class inheritance checking via base type traversal
• Self-exports (exporting the class's own type)

Implementation Details

• Analyzer ID: VSMEF003
• Severity: Warning (not error, as this may be intentional in some advanced scenarios)
• Category: Usage
• Targets: Classes with [Export(typeof(T))] attributes
• Type Compatibility: Uses Roslyn's INamedTypeSymbol and SymbolEqualityComparer for robust type checking

Testing

Added comprehensive test cases covering:

• Valid scenarios (no diagnostics): Interface implementation, class inheritance, self-exports
• Invalid scenarios (diagnostics): Unimplemented interfaces, unrelated classes
• Edge cases: Multiple exports, non-class types, contract name exports
• Both C# and VB.NET support

The core type compatibility logic was validated with a standalone test program confirming correct behavior for interface implementation and class inheritance detection.

Documentation

• Updated doc/analyzers/index.md with new analyzer entry
• Created detailed doc/analyzers/VSMEF003.md with examples and guidance
• Added entry to AnalyzerReleases.Unshipped.md
• Updated README with analyzer description

Fixes #79.

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