Support managed DLL launch and debugger attach

Parent

#10 — Add cross-platform VS Code editor adapter for MrBuild LSP proof

What to build

Add the second typed Coordinator launch mode: execute a managed DLL through a configurable dotnet executable. Provide actionable launch failures and a ready-made managed-process debugger attach configuration.

Acceptance criteria

• A managed Coordinator DLL launches as dotnet <dll> without shell parsing.
• The dotnet executable can be overridden for SDK/debugger selection.
• The spawned process is the actual managed Coordinator and remains attachable by the .NET debugger.
• The proof workspace includes an attach configuration that lets a developer select the Coordinator process.
• A missing DLL or missing dotnet executable produces a clear VS Code error and output message.
• Native executable launch remains supported.

Blocked by

• #12 — Launch the real Coordinator from VS Code

Start with the cross-platform VS Code editor adapter described in parent issue #10 and the Coordinator launch work in blocker #12. Trace how native executable launch is represented, then define the managed DLL and debugger-attach behavior against the acceptance criteria. Done means both launch modes work, failures are visible in VS Code and its output, and the proof workspace offers Coordinator process selection.