v0.9.1: Refine the idle whale silhouette and empty-state composition

[Hmbown]

Goal

Give the idle Codewhale mark one final art-direction pass so the whale is immediately recognizable, balanced, and charming without making the empty state louder or more decorative.

Agent-ready implementation

1. Start from current origin/main after PR Integrate CodeWhale v0.9.1 runtime and release surface #4675 is resolved. Read AGENTS.md and inspect IDLE_WHALE_ROWS plus empty-state composition and motion policy in crates/tui/src/tui/underwater.rs.
2. Refine the three-row terminal silhouette with careful attention to body proportion, head/tail readability, negative space, baseline, and centering. Prefer editing the existing mark over adding a new logo system.
3. Keep the whale in the existing human/brand color lane and preserve the current caustic shine, reduced-motion behavior, attention stillness, ASCII-safe fallback, and responsive visibility rules unless real visual evidence justifies a smaller simplification.
4. Review the whale together with its context line, Fleet setup action, ambient fish/bubbles, header, and composer. The whole empty state must remain calm.
5. Capture real PTY evidence at wide, standard, narrow, dark, light, reduced-motion, and ASCII-safe settings. Do not use generated or fake product screenshots.

Acceptance

• The silhouette reads as a whale at a glance and feels optically centered at supported sizes.
• No clipping, width drift, cursor movement, noisy animation, or competition with the primary Fleet action.
• Compact terminals still yield decorative content before functional content.
• Existing motion, accessibility, and empty-state tests remain green; add behavior-level snapshot or width tests for the final mark.
• Locked TUI tests, cargo fmt, git diff --check, and real PTY visual receipts pass.

Boundaries

Do not redesign the website logo, add image assets, or expand the empty state into a dashboard. Stop at a tested PR; do not publish or release.