[test-recorder] Dev-server reuse doesn't verify backend identity; readiness check only proves Vite responds

### Problem
Distribution selection is a suggestion, not an invariant:

- `src/devserver/manager.ts:24-50` + `src/commands/record.ts:619-633` — `ensureDevServer` reuses any ready Vite server on the port without verifying which npm script / `DEV_SERVER_COMFYUI_URL` it was started with. Selecting staging/prod/custom backend while a server for another backend is running only prints a warning and records against the wrong environment.
- `src/checks/devServer.ts:41-58` + `src/devserver/manager.ts:78-99` — readiness proves only that Vite answers HTTP; the recorder can launch into an error page before compilation/backend proxying is usable (`src/recorder/runner.ts:158-160`).

### Suggested fix
Expose dev-server identity (checkout + backend target) via a probe endpoint or launch metadata; on mismatch return a typed outcome requiring restart or explicit override. Extend readiness to poll an app endpoint plus the selected backend through the Vite proxy.

_Line references are against branch `cb/comfy-test-usability` (PR #15837); adjust after merge._