MVP alpha.11: Close gaps for beta

[ZebulonRouseFrantzich]

This pull request introduces several significant improvements to the CI pipeline and documentation, with a focus on formal security model checking, performance contract enforcement, and updated project status. The CI workflow now includes a dedicated formal security kernel job that conditionally runs TLA+ model checks based on PR changes, and a required shared-Linux performance contract gate. Documentation has been updated to reflect these new CI lanes, clarify the supported product surface, and provide more detail on security and attestation flows.

CI/CD Pipeline Enhancements:

• Added a formal-security-kernel job to the GitHub Actions workflow (.github/workflows/ci.yml) that conditionally runs TLA+ model checks (just model-check, just model-check-core) based on which files are changed in a PR, improving efficiency and ensuring formal verification coverage where needed.
• Introduced a required shared-Linux performance contract gate by renaming and updating the canonical Linux CI lane to ci-required-shared-linux, and clarified the matrix labels for CI jobs.
• Enabled workflow concurrency controls to prevent duplicate runs and cancel in-progress runs for PRs.

Documentation Updates:

• Updated README.md and CONTRIBUTING.md to document the new CI lanes (ci-required-shared-linux, ci-fast, model-check-core, model-check-replay), clarify that only the required shared-Linux performance contract subset is enforced on CI, and describe the new formal security kernel check. [1] [2] [3]
• Revised project status and component descriptions to reflect the current supported surface, more accurate attestation and evidence flows, and the addition of performance contracts and TLA+ model checking. [1] [2] [3] [4]

Minor Improvements:

• Added a new help output line for change_draft to the broker CLI core test, reflecting the default low-level start command.