Render layering: ratatui chrome over libghostty pane interiors

TL;DR. phux-client uses two renderers on disjoint screen regions. libghostty paints pane interiors on the hot path so kitty graphics, sixel, OSC 8, and the Kitty key protocol pass through unchanged. ratatui paints the chrome (status bar, dividers, modals); the layers composite rather than interleave. CI grep-guards the boundary.

---

Under epic phux-5ke (and TBD ADR-0020) phux-client uses two renderers for disjoint screen regions. libghostty paints pane interiors on the hot path — kitty graphics, sixel, OSC 8 hyperlinks, and the Kitty key protocol all pass through unchanged. ratatui paints the chrome: status bar, pane dividers, borders, modals, future tab bar. The layers composite rather than interleave; chrome carves skip-cell rectangles for pane rects so libghostty owns those cells exclusively.

The ratatui dependency is scoped to crates/phux-client/src/render/ (submodules chrome and overlay). Attach loop, pane mirror, predict layer, and layout math stay ratatui-free. A CI grep guard, scripts/check-ratatui-boundary.sh, runs from just ci and fails the build if a ratatui import appears outside render/.