Phase C — duplication cleanup: editor primitive + chrome lifts (umbrella)

[JDonaghy]

Umbrella issue tracking the Phase C — duplication cleanup arc planned in docs/PHASE_C_PLAN.md.

The B.5c → #271 arc closed the trait surface and runner-crate vision. What's still duplicated between the vcd (TUI) and vimcode (GTK) binaries is:

1. The editor "widget" itself — tui_main/render_impl.rs::render_window (~600 LOC) and gtk/draw.rs::draw_window (~800 LOC) paint the same RenderedWindow data into different surfaces.
2. Five remaining sidebar surfaces — debug, source control, extension, AI, settings chrome — plus the scrollbar paint.

Phase C lifts the editor paint into a quadraui::Editor primitive (Phase 1 only — engine slice deferred) and lifts the easy chrome (scrollbar / settings header / AI sidebar). Three remaining surfaces (extension, debug, source control) deferred as separate follow-up issues.

Stages

• Stage 1 — quadraui::Editor primitive + TUI/GTK rasterisers — sub-issue
• Stage 2 — quadraui::Scrollbar primitive — sub-issue
• Stage 3 — Settings panel chrome helper — sub-issue
• Stage 4 — AI sidebar MessageList primitive — sub-issue

Predecessor / successor

• Predecessor: B.5c → B.5e → Phase B.5c followup — lift rich_text_popup / completions / find_replace into quadraui::{tui,gtk} #266 → Phase B.5c followup — Backend::draw_dialog needs dual-Pango-layout handling #267 → Phase B.5e followup — quadraui::gtk::run runner + multi-DrawingArea support #270 → Phase B.5c followup — lift find_replace into quadraui::{tui,gtk} (primitive migration) #271 (all closed)
• Successor: B.6 (Win-GUI rebuild) — moves to after Phase C so it inherits the editor paint shape automatically once quadraui::win_gui::draw_editor lands

Deferred follow-ups (filed separately)

• Extension panel lift (~12 hrs, depends on extending TreeView with section headers)
• Debug sidebar lift (~16 hrs, four sections + per-section scrollbars; hand-rolled hit-test per ContextMenu: motion handlers should use primitive's hit_test, not hand-rolled row math #210)
• Source control panel lift (~24 hrs, complex due to inline commit-message editing)
• Phase 2 editor engine-slice extraction — separate multi-month wave

[JDonaghy]

All four Phase C stages shipped (#276 + #277 + #278 + #279). The deferred follow-ups (#280, #281, #282) also shipped, with #282 already on TreeView before the explicit lift was needed. TUI/GTK paint duplication arc closed end-to-end as of develop@7f3498c.