Implement automatic dispose code generation

[konard]

Summary

Implements automatic dispose code generation feature as requested in issue #2. This feature allows classes inheriting from DisposableBase to automatically generate disposal code for all disposable fields using C# source generators.

Key Features

• AutoDispose Attribute: Apply [AutoDispose] to classes that need automatic disposal
• Source Generator: Compile-time code generation with zero runtime overhead
• Smart Field Detection: Automatically finds all fields implementing IDisposable or Platform.Disposables.IDisposable
• Readonly Field Support: Correctly handles readonly fields by skipping null assignment
• Null-Safe Disposal: Uses null-conditional operator (?.) for safe disposal

Usage Example

[AutoDispose]
public partial class MyClass : DisposableBase
{
    private readonly FileStream _readOnlyField;
    private MemoryStream _writableField;
    
    // Generated Dispose method will automatically:
    // - Call _readOnlyField?.Dispose()
    // - Call _writableField?.Dispose()
    // - Set _writableField = null (non-readonly only)
}

Changes Made

• Added AutoDisposeAttribute class for marking classes
• Created AutoDisposeSourceGenerator with full compile-time analysis
• Added comprehensive test suite with edge cases
• Included documentation and examples
• Updated project version to 0.5.0
• Fixed compatibility issue with existing tests (net7 → net8)

Test Plan

• All existing tests pass
• New source generator tests verify functionality
• Tests cover readonly fields, null fields, and disposal order
• Verified compile-time code generation works correctly

🤖 Generated with Claude Code

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Resolves #2