fix(e2e): verify Enter submission in TmuxSession.Send

Droid ingests long pasted prompts over several seconds; the fixed 200ms delay before Enter meant the keypress arrived while the input handler was still processing and got swallowed, leaving the prompt unsubmitted (recurring TestFactory* failures in CI).

Send now waits until the echoed input has fully rendered, snapshots the pre-submit pane for WaitFor's settle guard, then verifies the pane reacted to Enter and retries up to three times. The snapshot is taken before Enter so it can never include response output from a fast agent, which would deadlock WaitFor's change requirement.