R3: isolated Takeover Twin and trusted fixture runner

[yava-code]

Outcome

Builds the executable R3 boundary for one reviewed fixture: a sanitized standalone participant repo, opaque twin lifecycle, immutable command registry, extension-owned catalog, and exact-state runner.

Evidence

• 50/50 extension tests pass locally on Windows
• TypeScript check, production build, and VSIX packaging pass
• fixture assets are shipped in the VSIX
• production files/index/HEAD/refs/remotes/worktrees remain unchanged
• descendant process-tree termination is exercised in a path with spaces

Boundary

This is not an arbitrary-code sandbox. Workspace, participant-authored, and non-catalog execution remain unsupported until the R7 backend gate.

[gemini-code-assist]

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[coderabbitai]

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[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.