fix(ui): widen arcade stats bar layout chunk to fit both HUD lines

[bug-ops]

Summary

• render_stats_bar builds a bordered Paragraph with 2 content lines (SCORE/LIVES/MULT/grace, and COMBO/STREAK/level/BEST), which needs height 4 (1 top border + 2 content + 1 bottom border). All 3 call sites (render_countdown, render_playing, render_paused in src/ui/render/minigame.rs) allocated only Constraint::Length(3), silently clipping the second HUD line at every terminal size.
• Widened all 3 to Length(4).
• Added a regression test per screen, asserting COMBO/STREAK/BEST content is actually present in the rendered buffer (not just "doesn't panic").
• Live-verified via tmux (130x45, cargo run --no-default-features): both HUD lines now render correctly in Arcade Playing.

Note on layout slack

At the documented 80x24 minimum terminal size, render_playing's fixed chunks now total 13 rows (3+3+3+4), which leaves the key-history popup's visibility check passing with exactly zero margin (previously 1 row of slack) — see the comment added at src/ui/render/minigame.rs:154. Not a regression at 80x24 (verified, and covered by the existing test_render_playing_key_history_popup_does_not_corrupt_target_panel_border test), but this repo has hit this exact boundary before (#292, #385/#388) — flagging so future changes to this layout re-check the math before growing any other fixed chunk in it.

Test plan

• cargo +nightly fmt --check
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo nextest run --workspace --all-features --lib --bins (2220/2220 passed)
• Live tmux repro re-verified: both HUD lines render, no clipping, no border corruption

Fixes #393