quadraui: lift debug sidebar (4 sections + per-section scrollbars)

[JDonaghy]

Deferred from Phase C (#275). One of three remaining sidebar surfaces still hand-rolled per backend.

Goal

Migrate the Debug sidebar (Call Stack / Variables / Watch / Breakpoints) to paint through quadraui primitives. Currently bespoke in both backends with hand-rolled hit math (#210, #211).

Why deferred

Four sections + per-section vertical scrollbars + nested expandable variable trees. Hit-test is hand-rolled per #210 (motion handlers should call layout.hit_test() directly, but the wires from "mouse moved" → "selected_idx changed" are still per-backend). Multiple bugs across B.5 traced back to this same shape.

Lifting needs:

1. quadraui::TreeView extension to support multiple sections with per-section scroll state, OR a new MultiSectionView primitive composing several TreeViews.
2. Resolution of ContextMenu: motion handlers should use primitive's hit_test, not hand-rolled row math #210 (motion handlers route through layout hit_test).

Estimated effort

~16 hrs

References

• TUI: src/tui_main/panels.rs — render_debug_sidebar
• GTK: src/gtk/draw.rs::draw_debug_sidebar
• DebugSidebarData in src/render.rs
• Related: ContextMenu: motion handlers should use primitive's hit_test, not hand-rolled row math #210 (motion handlers should use layout hit_test), GTK debug panel: variable tree clicks land on row ~2 above the clicked one #211 (GTK debug variable tree clicks land 2 rows above), TUI debug panel: only 'local: args' is expandable; new variables created mid-session aren't #212 (TUI variables not expandable mid-session), TUI debug sidebar: scrollbar click doesn't reach handle_mouse #242 + GTK debug sidebar: no GestureDrag for scrollbar — thumb-drag acts as track click #243 (debug sidebar scrollbar bugs)

[JDonaghy]

Implemented in commits f3d78d6 + b7b2cfc + d06568c + 38c052e + c0fdc8a + 33cfd2b + f15a490 + 7f3498c (Path A — ff-merge to develop). Smoke surfaced one pre-existing issue still open: #292 (GTK F-keys not reaching debugger when debug sidebar has focus — likely GTK menu-bar activator interception).