Modernize codebase to C# 13

[Copilot]

Upgrades the repository to use modern C# language features while maintaining backward compatibility.

Changes

Framework & Language

• Upgraded .NET 6.0 → .NET 9.0 (eliminates EOL warnings)
• Set LangVersion to C# 13

File-scoped Namespaces (C# 10)

// Before
namespace DiffPlex.Model
{
    public class DiffResult { ... }
}

// After
namespace DiffPlex.Model;

public class DiffResult { ... }

Primary Constructors (C# 12)

// Before
public class DiffBlock
{
    public int DeleteStartA { get; }
    public int DeleteCountA { get; }
    
    public DiffBlock(int deleteStartA, int deleteCountA)
    {
        DeleteStartA = deleteStartA;
        DeleteCountA = deleteCountA;
    }
}

// After
public class DiffBlock(int deleteStartA, int deleteCountA)
{
    public int DeleteStartA { get; } = deleteStartA;
    public int DeleteCountA { get; } = deleteCountA;
}

Collection Expressions (C# 12)

// Before
public List<DiffPiece> Lines { get; } = new List<DiffPiece>();
private readonly string[] lineSeparators = new[] {"\r\n", "\r", "\n"};

// After
public List<DiffPiece> Lines { get; } = [];
private readonly string[] lineSeparators = ["\r\n", "\r", "\n"];

Target-typed New (C# 9)

// Before
public static Differ Instance { get; } = new Differ();

// After
public static Differ Instance { get; } = new();

Impact

• 27 source files modernized
• Zero breaking changes (multi-targeting preserved: net45, netstandard2.0, net9.0)
• All 248 tests passing

Original prompt

Upgrade this repo to use the most recent C# features

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