Trace: verify Pi interactive welcome message #3722

Problem

The Pi welcome-message matrix cell is Unknown. A prior Pi 0.80.10 TUI smoke showed a banner, but the local headless harness cannot assert this UI-only notification.

Scope

Verify the interactive Pi session-start notification in local Trace mode, without changing production behavior unless the test exposes a defect.

Acceptance criteria

• Drive a pinned Pi TUI in a disposable repository with Trace local mode and capture the visible session-start notice.
• Assert the notice says the session is recorded and identifies the on-disk evidence destination.
• Confirm headless JSONL remains machine-readable and does not claim to show the UI-only notice.
• Add a deterministic verification procedure/test where the Pi TUI signal is observable, and update only the welcome-message Pi matrix cell when verified.

Non-goals

• Credentials validation, post-push links, or control-plane attestation.
• Claiming a headless model-context message is a user-visible TUI notification.

Split from #3718; baseline Pi support #3704.