feat(Workspace.Roslyn): Add NullRoslynWorkspace and ActiveWorkspaceProxy

- NullRoslynWorkspace: Null object pattern that throws helpful errors
  when accessed without a loaded solution
- ActiveWorkspaceProxy: Delegates to IWorkspaceManager.ActiveWorkspace
  for multi-workspace support

These enable the MCP server to start without a solution loaded,
providing clear error messages when tools are used.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Mike

src/FractalDataWorks.Workspace.Roslyn/ActiveWorkspaceProxy.cs

@@ -0,0 +1,86 @@
+using System;
+using System.Collections.Generic;
+using FractalDataWorks.Results;
+using Microsoft.CodeAnalysis;
+
+namespace FractalDataWorks.Workspace.Roslyn;
+
+/// <summary>
+/// A proxy implementation of <see cref="IRoslynWorkspace"/> that delegates to the active workspace
+/// from the workspace manager. This allows singleton tools to always use the current active workspace.
+/// </summary>
+public sealed class ActiveWorkspaceProxy : IRoslynWorkspace
+{
+    private readonly IWorkspaceManager _workspaceManager;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="ActiveWorkspaceProxy"/> class.
+    /// </summary>
+    public ActiveWorkspaceProxy(IWorkspaceManager workspaceManager)
+    {
+        _workspaceManager = workspaceManager ?? throw new ArgumentNullException(nameof(workspaceManager));
+    }
+
+    private IRoslynWorkspace GetWorkspace() =>
+        _workspaceManager.ActiveWorkspace
+        ?? throw new InvalidOperationException("No solution is loaded. Use the OpenSolution tool to load a solution first.");
+
+    /// <inheritdoc/>
+    public Solution CurrentSolution => GetWorkspace().CurrentSolution;
+
+    /// <inheritdoc/>
+    public Solution? BaselineSolution => GetWorkspace().BaselineSolution;
+
+    /// <inheritdoc/>
+    public Solution Current => GetWorkspace().Current;
+
+    /// <inheritdoc/>
+    public Solution? Baseline => GetWorkspace().Baseline;
+
+    /// <inheritdoc/>
+    public int SnapshotCount => _workspaceManager.ActiveWorkspace?.SnapshotCount ?? 0;
+
+    /// <inheritdoc/>
+    public bool HasChanges => _workspaceManager.ActiveWorkspace?.HasChanges ?? false;
+
+    /// <inheritdoc/>
+    public void UpdateSolution(Solution solution) => GetWorkspace().UpdateSolution(solution);
+
+    /// <inheritdoc/>
+    public void Update(Solution state) => GetWorkspace().Update(state);
+
+    /// <inheritdoc/>
+    public void SetBaseline(Solution state) => GetWorkspace().SetBaseline(state);
+
+    /// <inheritdoc/>
+    public string CreateSnapshot(string name, string description) =>
+        GetWorkspace().CreateSnapshot(name, description);
+
+    /// <inheritdoc/>
+    public IGenericResult<Solution> RestoreSnapshot(string snapshotId) =>
+        GetWorkspace().RestoreSnapshot(snapshotId);
+
+    /// <inheritdoc/>
+    public IEnumerable<SnapshotInfo> ListSnapshots() =>
+        _workspaceManager.ActiveWorkspace?.ListSnapshots() ?? [];
+
+    /// <inheritdoc/>
+    public bool RemoveSnapshot(string snapshotId) =>
+        _workspaceManager.ActiveWorkspace?.RemoveSnapshot(snapshotId) ?? false;
+
+    /// <inheritdoc/>
+    public void ClearSnapshots() => _workspaceManager.ActiveWorkspace?.ClearSnapshots();
+
+    /// <inheritdoc/>
+    public IReadOnlyDictionary<string, string> GetChangesFromBaseline() =>
+        _workspaceManager.ActiveWorkspace?.GetChangesFromBaseline()
+        ?? new Dictionary<string, string>(StringComparer.Ordinal);
+
+    /// <inheritdoc/>
+    public IReadOnlyDictionary<string, string>? GetChangesFromSnapshot(string snapshotId) =>
+        _workspaceManager.ActiveWorkspace?.GetChangesFromSnapshot(snapshotId);
+
+    /// <inheritdoc/>
+    public void ApplyDocumentChanges(IReadOnlyDictionary<string, string> documentChanges) =>
+        GetWorkspace().ApplyDocumentChanges(documentChanges);
+}

src/FractalDataWorks.Workspace.Roslyn/NullRoslynWorkspace.cs

@@ -0,0 +1,80 @@
+using System;
+using System.Collections.Generic;
+using FractalDataWorks.Results;
+using Microsoft.CodeAnalysis;
+
+namespace FractalDataWorks.Workspace.Roslyn;
+
+/// <summary>
+/// A null object implementation of <see cref="IRoslynWorkspace"/> that throws helpful errors when accessed.
+/// </summary>
+/// <remarks>
+/// This allows the MCP server to start without a solution loaded. Tools will fail gracefully
+/// at execution time with a clear message to use OpenSolution first.
+/// </remarks>
+public sealed class NullRoslynWorkspace : IRoslynWorkspace
+{
+    /// <summary>
+    /// Gets the singleton instance of the null workspace.
+    /// </summary>
+    public static NullRoslynWorkspace Instance { get; } = new();
+
+    private NullRoslynWorkspace() { }
+
+    private static InvalidOperationException NoWorkspaceError() =>
+        new("No solution is loaded. Use the OpenSolution tool to load a solution first.");
+
+    /// <inheritdoc/>
+    public Solution CurrentSolution => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public Solution? BaselineSolution => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public Solution Current => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public Solution? Baseline => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public int SnapshotCount => 0;
+
+    /// <inheritdoc/>
+    public bool HasChanges => false;
+
+    /// <inheritdoc/>
+    public void UpdateSolution(Solution solution) => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public void Update(Solution state) => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public void SetBaseline(Solution state) => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public string CreateSnapshot(string name, string description) => throw NoWorkspaceError();
+
+    /// <inheritdoc/>
+    public IGenericResult<Solution> RestoreSnapshot(string snapshotId) =>
+        GenericResult<Solution>.Failure("No solution is loaded. Use the OpenSolution tool to load a solution first.");
+
+    /// <inheritdoc/>
+    public IEnumerable<SnapshotInfo> ListSnapshots() => [];
+
+    /// <inheritdoc/>
+    public bool RemoveSnapshot(string snapshotId) => false;
+
+    /// <inheritdoc/>
+    public void ClearSnapshots() { }
+
+    /// <inheritdoc/>
+    public IReadOnlyDictionary<string, string> GetChangesFromBaseline() =>
+        new Dictionary<string, string>(StringComparer.Ordinal);
+
+    /// <inheritdoc/>
+    public IReadOnlyDictionary<string, string>? GetChangesFromSnapshot(string snapshotId) => null;
+
+    /// <inheritdoc/>
+    public void ApplyDocumentChanges(IReadOnlyDictionary<string, string> documentChanges) =>
+        throw NoWorkspaceError();
+}