Add cross-version interop test against the published ext-apps 1.7.5 (#770)

Runs the published 1.7.5 App and AppBridge against the 2.x build over an in-memory transport in both pairings, via an npm alias devDependency and sdk 1.30. Complements wire-compat.test.ts: this one runs the real 1.x code and is the only test that catches a coordinated 2.x wire rename.