An unknown edge sign-in creates no account and goes to the forge's own signup (with the createUser door list updated on purpose) #2763

Replaces #2593. Both earlier pulls were stopped by test/signup-gate-parent.test.ts > "every caller of createUser says where its account comes from". That test lists "src/core/users.ts": ["edge"] as a way in, and it is meant to change when a way in is removed.

Done:

• [ ] ensureEdgeUser (src/core/users.ts) only finds: an existing active user is returned, and an unknown login returns undefined and creates nothing
• [ ] that request is treated as signed out, and the page offers /signup with the signup setting applied. With signup closed it says so and never lets the person in
• [ ] accounts the edge already made still sign in
• [ ] remove "src/core/users.ts": ["edge"] from the expected callers in test/signup-gate-parent.test.ts, and say in the pull that the test changed because a door was closed on purpose
• [ ] tests: an unknown edge login creates no user row, an edge-made account still signs in, and with signup closed an unknown login reads "signup is closed"

Run test/signup-gate-parent.test.ts and the edge tests before pushing.