Raise welcome animation breakpoint to 37 rows

[mzeng-openai]

Motivation

• The large ASCII welcome animation can push onboarding content below the fold on default-height terminals, making the CLI appear unresponsive; raising the breakpoint prevents that.
• The existing test measured an arbitrary row count rather than asserting the welcome line position relative to the animation frame, which made the intent unclear.

Description

• Increase MIN_ANIMATION_HEIGHT from 20 to 37 in codex-rs/tui/src/onboarding/welcome.rs so the animation is skipped unless there is enough vertical space.
• Replace the brittle measurement logic in the welcome render test with a row_containing helper and assert the welcome row equals the frame height plus the spacer line (frame_lines + 1).
• Add a regression test welcome_skips_animation_below_height_breakpoint that verifies the animation is not rendered when the viewport height is one row below the breakpoint.

Testing

• Ran formatting with ~/.cargo/bin/just fmt which completed successfully.
• Ran unit tests for the crate with cargo test -p codex-tui --lib and they passed (unit test suite succeeded).
• Ran cargo test -p codex-tui which reported a failing integration test in this environment because the test cannot locate the codex binary, so full crate tests are blocked here (environment limitation).

---

Codex Task