docs: clarify macOS old-core health status fallback

[mvanhorn]

Description

Fixes #1432

Docs-only change to specs/109-ux-navigation-consistency/contracts/health-vocabulary.md.

The health vocabulary contract said every core that sends no status falls back to the legacy words. That does not match the macOS app. ServerStatusLinePresentation.line(for:) only derives legacy words after two earlier branches fail: a present non-empty health.status, then a present health object with a non-empty summary. So on macOS, an older core that sends a summary such as Connection refused and no status shows that summary as the whole label on the Servers row, the tray server submenu, and the Server Detail header.

The paragraph now describes the three branches in order and names testOldCoreHealthWithoutStatusUsesItsSummary and testOldCoreFallbacksUseLegacyWordsOnlyWithoutHealth as the tests that pin this behavior. No code or tests change.

Verification:

cd native/macos/MCPProxy
swift test --filter ServerStatusLineTests

Executed 13 tests, with 0 failures, confirming the behavior the paragraph now describes.

Testing

• I have tested these changes locally
• I have added/updated tests that prove my fix is effective or my feature works
• All existing tests pass

The full suite was not run, since only a markdown file changes.

[Dumbris]

Thanks a lot @mvanhorn! The contract now matches ServerStatusLinePresentation.line(for:) exactly, and pointing at the two tests that pin the behavior makes it easy to keep it accurate. Much appreciated — this will merge as soon as CI finishes.