Add the shared fixture and isolated VS Code proof launcher

Parent

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

What to build

Create the editor-neutral mixed-language proof fixture and the reproducible VS Code proof launcher. The launcher must create disposable user-data and extension directories, load only the proof adapter, open the fixture workspace, and report enough instance/profile/process information for editor-driven automation and debugging.

Acceptance criteria

• The fixture contains selected F# and C# projects suitable for bidirectional source navigation.
• The fixture contains deterministic source that produces an intentional language diagnostic.
• The launcher creates disposable VS Code user-data and extensions directories.
• The launcher opens only the shared fixture workspace with the proof adapter enabled.
• The launcher is cross-platform and uses the existing build pipeline for prerequisites.
• The launcher reports the VS Code instance, profile, fixture, and relevant process information.
• Repeated runs do not depend on prior editor state or unrelated installed extensions.

Blocked by

• #12 — Launch the real Coordinator from VS Code

Start with parent issue #10 and blocked issue #12, then inspect the existing build pipeline for prerequisite and VS Code integration conventions. Define the shared F# and C# fixture and launcher around the listed acceptance criteria; done means repeated cross-platform runs use isolated editor state, load only the proof adapter, and report the requested instance information.