Keep unreachable peer builds unknown

[myobie]

What changed

• Include unreachable peers in the build-agreement check.
• Report every unverified peer build as unknown, including roaming peers.
• Derive the closing summary as a testable value, so unknown results cannot print nothing to do.
• Keep the existing build-risk explanation.

Red proof

Before the production change, both new unreachable-build tests failed. The build verdict was ok instead of unknown.

Verification

• cargo test --lib doctor::tests — 22 passed
• cargo test --lib — 469 passed, 4 ignored
• cargo check --all-targets — passed
• git diff --check — passed

No release or deployment is part of this pull request.