Phase C Stage 1: lift quadraui::Editor primitive + TUI/GTK rasterisers

[JDonaghy]

Part of #275 (Phase C umbrella).

Goal

Lift the editor "widget" paint into a published quadraui::Editor primitive + dual rasterisers, so vimcode's tui_main/render_impl.rs::render_window and gtk/draw.rs::draw_window collapse to delegators. Net -1500 to -2000 LOC vimcode-private paint code. Phase 1 only — engine slice extraction explicitly deferred to a separate later wave.

Scope

New module quadraui/src/primitives/editor.rs:

• pub struct Editor mirroring render::RenderedWindow. Carries lines: Vec<EditorLine>, cursor: Option<EditorCursor>, scroll_top: usize, scroll_left: usize, gutter_chars: usize, selection: Option<Selection>, find_matches: Vec<Match>, optional dap_current_line, etc.
• pub struct EditorLine mirroring render::RenderedLine: spans: Vec<StyledSpan>, gutter_text, git_status, is_cursorline, is_diff, is_dap_current, bp_state, diagnostic, fold_marker, virtual_text, indent_guides: Vec<u16>, color_columns: Vec<u16>, wrap_continuation: bool, etc.
• Supporting types: EditorCursor, Selection, BreakpointState, GitLineStatus, DiagnosticSeverity. Lift from render.rs or reuse from quadraui::types.

New rasterisers:

• quadraui/src/tui/editor.rs::draw_editor(buf, rect, editor, theme) — verbatim port of tui_main/render_impl.rs::render_window body. Same paint categories in same order: bg fills (cursorline / diff / DAP-stopped) → gutter (BP, git, line numbers, diagnostic icon, lightbulb, fold marker) → text spans → indent guides → color columns → virtual text → diagnostic underlines → spell underlines → bracket-match highlights → selection overlay → yank-flash → scrollbars.
• quadraui/src/gtk/editor.rs::draw_editor(cr, layout, rect, editor, theme, line_height, char_width) — verbatim port of gtk/draw.rs::draw_window.

Theme growth: 2-4 new fields on quadraui::Theme for editor-specific colors (cursorline_bg, diff line tints, BP marker colors). Mapped from vimcode's rich render::Theme via q_theme() adapter. Matches the +5 fields #266 added.

Vimcode adoption:

• src/render.rs — add build_editor_primitive(engine, window_id) -> quadraui::Editor builder filling the primitive from existing engine state. RenderedWindow keeps existing for transition compatibility.
• src/tui_main/render_impl.rs::render_window — body collapses to quadraui::tui::draw_editor(buf, rect, &editor, &theme).
• src/gtk/draw.rs::draw_window — body collapses similarly.
• src/tui_main/quadraui_tui.rs (q_theme), src/gtk/quadraui_gtk.rs (q_theme) — populate new theme fields.

Untouched: click/mouse handlers (operate on engine state directly). Win-GUI editor (waits for B.6). Engine slice extraction (Phase 2, separate wave).

Reuse principle

RenderedWindow / RenderedLine field shapes are the source of truth. Don't redesign — translate verbatim. The TUI tests in core::engine::tests exercise editor state through to RenderedWindow; they continue to validate end-to-end via the new primitive without modification.

Verification

cargo build --no-default-features
cargo build
cargo clippy --no-default-features -- -D warnings
cargo clippy -- -D warnings
cargo test --no-default-features
cd quadraui && cargo test --features tui --features gtk
cd .. && cargo build -p kubeui && cargo build -p kubeui-gtk
cd quadraui && cargo build --example tui_app --features tui && cargo build --example gtk_app --features gtk

Smoke: open vimcode + vcd, navigate (j/k/h/l, page up/down, scroll), verify cursor / line numbers / git diff markers / syntax colors / selection / diagnostic squiggles / cursorline / scrollbars / wrap continuation render identically before vs after. Open a file with active LSP diagnostics + git changes + a fold to exercise the harder paint categories.

[JDonaghy]

Implemented in PR #284 (merged)