fix(tui): restore reliable mouse and responsive layout - #49

Summary

• require a semantic primary press and matching release, cancel stale owners, and route pointer input to the topmost visible overlay
• make responsive root, body, footer, picker, modal, and management geometry usable at exact and zero-size boundaries
• keep Unicode grapheme, tab, selection, and caret behavior correct while bounding multiline rendering to the visible viewport, including content beyond 65,535 rows
• make terminal redraws selective and verify real SGR mouse input through a final-grid PTY oracle

Validation

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets --all-features -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo doc --locked --workspace --all-features --no-deps
• fresh isolated workspace tests/examples plus every benchmark under aggregate LLVM coverage
• bash scripts/check_coverage.sh /tmp/skit-tui-mouse-pr-final-v2.lcov — complete executable-source line coverage
• survivor-driven mutation scans, exact current reruns, and line-level fault injection for the final overlay and viewport changes
• cargo deny --locked check, cargo audit --deny warnings, Zizmor, and the documentation type/build/link checks

Scope

The UI model walker is intentionally not included here. PR #48 remains separate and untouched.