Route stroke frame-face traversal through helper

docs/modernization/debt.md

@@ -18,6 +18,10 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw` current and
+  dual stroke frame-face traversal now routes through the retained stroke
+  execution helper; framebuffer binding, shader uniform timing, dirty-box
+  mutation, sampler/texture binding, and live draw execution remain retained.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_compute`
   frame planning now routes brush-quad construction, mixer feedback bounds,
   2D/3D projection selection intent, and frame assembly through the retained

docs/modernization/roadmap.md

@@ -3087,6 +3087,10 @@ Results:
   execution helper for brush-quad construction, mixer feedback bounds, 2D/3D
   projection selection intent, and frame assembly, while legacy projection
   geometry, stroke samples, and live draw execution remain retained.
+- `Canvas::stroke_draw` current and dual stroke frame-face traversal now shares
+  the retained stroke execution helper, while framebuffer binding, shader
+  uniform timing, dirty-box mutation, sampler/texture binding, and live draw
+  execution remain retained.
 - Remaining simple color, hue, color-quad, grid heightmap, and pen/line
   preview shader setup in UI nodes and canvas modes now shares retained helper
   surfaces, while geometry, texture/sampler binding, blend/depth state,

src/canvas.cpp

@@ -690,18 +690,15 @@ void Canvas::stroke_draw()
     std::array<glm::vec4, 6> box_face = SIXPLETTE(glm::vec4(m_size, 0, 0));
     std::array<bool, 6> box_dirty = SIXPLETTE(false);
     glm::vec4 pad_color;
-    for (auto& f : frames)
-    {
+    pp::panopainter::execute_legacy_canvas_stroke_frame_faces(
+        frames,
+        [&](auto& f) {
             if (brush->m_tip_mix > 0.f)
             {
                 stroke_draw_mix(xy(f.m_mixer_rect), zw(f.m_mixer_rect));
             }
-
-        for (int i = 0; i < 6; i++)
-        {
-            auto& P = f.shapes[i];
-            if (P.size() < 3)
-                continue;
+        },
+        [&](auto& f, int i, auto& P) {
             m_dirty_face[i] = true;
             merge_faces[i] = true;
             box_dirty[i] = true;
@@ -731,8 +728,7 @@ void Canvas::stroke_draw()
             box_face[i] = dirty_update.pass_dirty_box;
             // TODO: maybe average color?
             pad_color = f.col;
-        }
-    }
+        });
 
     set_active_texture_unit(3);
     m_mixer.unbindTexture();
@@ -812,19 +808,17 @@ void Canvas::stroke_draw()
             dual_brush->m_tip_texture->bind() :
             unbind_texture_2d();
         auto frames_dual = stroke_draw_compute(*m_dual_stroke);
-        for (auto& f : frames_dual)
-        {
+        pp::panopainter::execute_legacy_canvas_stroke_frame_faces(
+            frames_dual,
+            [&](auto& f) {
                 pp::panopainter::apply_legacy_stroke_sample_uniforms(
                     pp::panopainter::LegacyStrokeSampleUniforms {
                         .color = f.col,
                         .alpha = f.flow,
                         .opacity = f.opacity,
                     });
-            for (int i = 0; i < 6; i++)
-            {
-                auto& P = f.shapes[i];
-                if (P.size() < 3)
-                    continue;
+            },
+            [&](auto&, int i, auto& P) {
                 m_tmp_dual[i].bindFramebuffer();
                 auto box_sample = stroke_draw_samples(i, P, copy_stroke_destination);
                 m_tmp_dual[i].unbindFramebuffer();
@@ -840,8 +834,7 @@ void Canvas::stroke_draw()
                             stroke_material.composite_pass.dual_blend_mode == 0,
                     });
                 m_dirty_box[i] = dirty_update.accumulated_dirty_box;
-            }
-        }
+            });
     }
 
     m_sampler_brush.unbind();

src/legacy_canvas_stroke_execution_services.h

@@ -8,6 +8,7 @@
 #include "util.h"
 
 #include <array>
+#include <cstddef>
 #include <functional>
 #include <span>
 #include <string_view>
@@ -152,6 +153,27 @@ template <typename ProjectStroke, typename MakeFrame>
     return frames;
 }
 
+template <typename Frames, typename BeginFrame, typename ExecuteFace>
+std::size_t execute_legacy_canvas_stroke_frame_faces(
+    Frames&& frames,
+    BeginFrame&& begin_frame,
+    ExecuteFace&& execute_face)
+{
+    std::size_t executed_faces = 0;
+    for (auto& frame : frames) {
+        begin_frame(frame);
+        for (int face_index = 0; face_index < 6; ++face_index) {
+            auto& vertices = frame.shapes[face_index];
+            if (vertices.size() < 3) {
+                continue;
+            }
+            execute_face(frame, face_index, vertices);
+            ++executed_faces;
+        }
+    }
+    return executed_faces;
+}
+
 [[nodiscard]] inline pp::paint_renderer::CanvasStrokeBox legacy_canvas_stroke_box(glm::vec4 box) noexcept
 {
     return pp::paint_renderer::CanvasStrokeBox {