v0.9.0: finish underwater receipt settling and phase-aware ambient motion

[Hmbown]

Problem

The underwater TUI has a coherent still-state contract, but three finishing behaviors remain: receipt settling, phase-aware depth, and a one-shot fish response to active work. These must not reintroduce motion while waiting for input, reviewing an approval, or using reduced motion.

Expected behavior

• Completed work receipts settle into history with a short, deterministic cascade rather than popping into place.
• Ombre/depth treatment reflects the current phase without reducing text contrast or changing layout geometry.
• Fish perform a single flee response when active work begins, then return to ambient behavior without looping.
• Waiting, approval, stopped, and reduced-motion states remain still.
• Resize and narrow-terminal behavior do not jitter or shift ownership of interactive rows.

Likely files

• crates/tui/src/tui/ocean.rs
• crates/tui/src/tui/underwater.rs
• crates/tui/src/tui/motion/
• crates/tui/src/tui/history.rs
• docs/MOTION_CONTRACT.md

Acceptance criteria

• Each behavior is driven by explicit phase transitions and deterministic time inputs.
• No continuous animation occurs during waiting, approval, stopped, or reduced-motion states.
• Receipt settling preserves ordering, deduplication, and counts.
• 80x24 and 100x32 terminal captures show no layout jitter or contrast regression.
• Unit/snapshot coverage exercises normal and reduced-motion paths.

[Hmbown]

Resolved by #4361 at merge commit 9bfa466. Receipt settling, phase-aware depth and fish motion, reduced/still behavior, and the motion contract documentation are complete with focused UI/ocean/underwater coverage. Full hosted macOS and Windows suites are green.