feat(tui): animate startup header mascot

[fcoury-oai]

Why

Give the initial Codex TUI banner a recognizable mascot without depending on terminal image protocols or the separate ambient pets surface. The header should remain portable across terminals that can render styled Unicode text.

What Changed

• Add a generated 14x6 Unicode mascot frame set for working, thinking, and read-below, plus the approved idle reference frame.
• Randomly choose one startup animation when the header is created and preserve that same animation through the placeholder-to-configured-session handoff.
• Settle the mascot to the idle reference frame when startup animation completes or when the header is committed to transcript history.
• Fall back to the text-only session header when the terminal is too narrow for the mascot and existing header text.
• Add focused animation/layout coverage and refresh the affected TUI snapshots.

Debugging Notes

• ChatWidget::new selects startup motion once and stores the shared StartupAnimation.
• handle_session_configured copies that same state into the configured SessionInfoCell so metadata arrival does not restart or switch motion.
• ChatWidget::pre_draw_tick schedules redraws while the active header reports a mascot tick, then flushes settled configured session info into transcript history.
• HistoryCell::finalize_for_history removes transient visual state before an active cell is committed.
• SessionHeaderHistoryCell::display_lines first attempts the side-by-side mascot layout, then rebuilds text at full width if the terminal requires the text-only fallback.

How to Test

1. Start Codex with animations enabled using just c.
2. Confirm the initial session header shows a 14x6 Unicode mascot and animates during startup.
3. Restart Codex a few times and confirm startup randomly selects among the working, thinking, and read-below motions.
4. Wait for startup animation to finish and confirm the mascot settles to the same idle reference image.
5. Resize the terminal narrow enough that the mascot and text no longer fit together. Confirm the header falls back to the existing text-only layout without over-truncating the directory.
6. Start Codex with animations disabled and confirm the mascot renders statically from the first frame.

Targeted verification:

• cargo check -p codex-tui
• just fix -p codex-tui
• just test -p codex-tui passes the mascot coverage and snapshots. The suite currently also reports two unrelated guardian feature-flag test failures in app::tests::update_feature_flags_disabling_guardian_*.

[chatgpt-codex-connector]

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[etraut-openai]

Please gate the animation on the tui.animations flag. This prevents problems with screen readers.

[fcoury-oai]

Please gate the animation on the tui.animations flag. This prevents problems with screen readers.

It's already gated, try starting it with -c 'tui.animations=false'

[etraut-openai]

I like the polish and character this adds to the TUI. Unfortunately, I'm seeing that it doesn't look so great in many terminals.

It looks good in kitty and ghostty.

But it kind of falls apart in Terminal.app, the VS Code terminal, the Codex app terminal, and iTerm2:

I haven't tried on Linux or Windows.

[fcoury-oai]

I agree: It's a compromise. People that are using these terminals are probably already used to the gaps on nerdfonts and unicode characters in general.

I see two ways forward: release it and see if people dislike or complain about it or gate it to some terminals. I am in favor of just releasing it and gathering feedback.

Edit: I will test again on Windows and Linux.