quadraui MultiSectionView: ship horizontal axis rasterisers

[JDonaghy]

Summary

The quadraui::MultiSectionView primitive (#293) ships with axis: Axis::Vertical | Horizontal wired through its API + layout math, but only the vertical rasterisers are implemented in v1. Horizontal rasterisers are todo!() until a real use case arrives.

This issue tracks adding the horizontal rasterisers when needed.

Why deferred

Decided during the #293 design pass: wiring the axis field through the API and layout algorithm (main-axis / cross-axis terminology internally) is cheap and avoids a future API break. Painting horizontal in TUI + GTK is real work with zero confirmed consumers today, so it's not worth doing speculatively.

When to pick this up

Any of:

• A consuming app (kubeui, Postman clone, vimcode itself) wants a horizontal multi-section ribbon — e.g. top-of-window Edit / View / Insert ribbon with tool-grid bodies, or a horizontal multi-pane layout where each pane has its own header.
• A new primitive composes MultiSectionView and needs horizontal layout (rare).
• We want to demonstrate cross-axis parity in a quadraui demo or talk.

What's involved

• quadraui::tui::draw_multi_section_view — handle Axis::Horizontal (column-major rather than row-major; section headers become left columns; bodies extend rightward).
• quadraui::gtk::draw_multi_section_view — same, in Cairo.
• Divider rendering: vertical │ glyph in TUI, vertical line in GTK; drag direction swap.
• Hit-test: MultiSectionViewLayout already uses Rects, so hit-test code should be axis-agnostic — verify.
• Tests: mirror the vertical tests with axis: Horizontal and content-aware bounds assertions.

Effort estimate: ~4–6 hrs once a real consumer is identified.

References

• quadraui: MultiSectionView primitive — eliminate per-backend section-walk drift #293 — MultiSectionView primitive (parent)
• D-003 in quadraui/docs/DECISIONS.md — captures the "wire axis, ship vertical-only" decision