A test pins `navigationBranch`, the release smoke's read of the service worker's navigate branch #6751

src/release/smoke.ts decides whether /sw.js can answer a navigation with Response.error() by cutting out its navigate branch with navigationBranch, and nothing tests that cut.

Done when test/smoke.test.ts (or a new file beside it) proves:

• [ ] on the forge's real service worker source the branch is found and contains no Response.error().
• [ ] a source with no mode === 'navigate' gives null.
• [ ] a Response.error() inside a comment in the branch is not counted; one after the branch's return; is outside it; one inside it is found.

Test only; a bug it finds is fixed in the same pull.