Make one validated, versioned DiffGraph artifact the canonical CLI contract

[nia-sg-bot]

Why

JSON, terminal, HTML, VS Code, and website proof must consume one product contract rather than parallel analysis/rendering paths.

Acceptance criteria

• wild diff creates one validated canonical artifact, then routes it to JSON/terminal/HTML consumers.
• Publish and version the schema with explicit major-version compatibility behavior.
• Define stdout/stderr, exit codes, no-change behavior, diagnostics, output paths, cancellation, and offline behavior.
• Fail closed when canonical validation is unavailable or output is invalid.
• Provide representative golden artifacts consumed in CLI and extension CI.
• Consumers safely reject unsupported schema majors.
• Legacy HTML fallback has a documented removal window.
• No AI-derived symbols or relationships enter canonical output.

Related

CLI #20 and WildestAI/wildest-vscode-ext#17.