Contract tests against metaseed-hub (sole consumer)

[sorenwacker]

Part of #61 (Promises).

metaseed-hub is the sole external consumer. Today the rule is informal ('check ../metaseed-hub before renaming a public symbol'). Make it a test.

• A contract test suite exercising the public API the way hub uses it.
• Run it in CI so a breaking change fails before release.

Acceptance. A breaking change to a hub-used public symbol turns CI red.