Extract stroke mix shell setup helper

docs/modernization/debt.md

@@ -94,6 +94,9 @@ agent or engineer to remove them without reconstructing context from chat.
   `retained_stroke_mix_pass_shell_executor_preserves_combined_wiring`.
 - 2026-06-13: `LATER-003` was narrowed again. `Canvas::stroke_draw_mix()`
   no longer computes an unused mix-plane plan in the live shell path.
+- 2026-06-13: `LATER-003` was narrowed again. `Canvas::stroke_draw_mix()`
+  now routes the remaining framebuffer setup callbacks through a local helper,
+  leaving the method with only shell assembly and executor dispatch.
 - 2026-06-13: `DEBT-0036` was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes final composite execution and preview copy-back through a retained
   local wrapper, leaving the call site with only sequence wiring.

docs/modernization/tasks.md

@@ -638,6 +638,9 @@ Progress Notes:
 - 2026-06-13: `Canvas::stroke_draw_mix()` no longer computes an unused mix-plane
   plan in the live shell path; the remaining code is the retained shell setup
   and executor call.
+- 2026-06-13: `Canvas::stroke_draw_mix()` now routes the remaining framebuffer
+  setup callbacks through a local helper, leaving the method with only shell
+  assembly and executor dispatch.
 - 2026-06-13: `Canvas::stroke_draw_samples()` now reuses a retained destination
   texture dispatch helper for the live sample path; `Canvas` still owns the
   concrete face textures and callback execution.

src/canvas.cpp

@@ -392,24 +392,7 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
     gl.save();
     const auto layer_index = m_current_layer_idx;
     auto& current_layer = *m_layers[layer_index];
-    std::array<glm::mat4, 6> plane_transform {};
+    const auto mix_shell = make_canvas_stroke_mix_pass_shell(*this, bb_min, bb_sz);
-    std::copy(std::begin(m_plane_transform), std::end(m_plane_transform), plane_transform.begin());
-    const auto mix_shell = pp::panopainter::make_legacy_canvas_stroke_mix_pass_shell(
-        [&] {
-            m_mixer.bindFramebuffer();
-            apply_canvas_viewport(0, 0, m_mixer.getWidth(), m_mixer.getHeight());
-            apply_canvas_capability(depth_test_state(), false);
-            apply_canvas_capability(scissor_test_state(), true);
-            apply_canvas_capability(blend_state(), false);
-            apply_canvas_scissor(
-                static_cast<std::int32_t>(bb_min.x),
-                static_cast<std::int32_t>(bb_min.y),
-                static_cast<std::int32_t>(bb_sz.x),
-                static_cast<std::int32_t>(bb_sz.y));
-        },
-        [&] {
-            m_mixer.unbindFramebuffer();
-        });
     [[maybe_unused]] const auto mix_result = pp::panopainter::execute_legacy_canvas_stroke_mix_pass_shell(
         mix_shell.setup.begin,
         mix_shell.setup.end,
@@ -527,6 +510,29 @@ static void execute_canvas_draw_merge_final_plane_composite(
     pp::panopainter::execute_legacy_canvas_draw_merge_final_plane_composite(uniforms, execution);
 }
 
+static auto make_canvas_stroke_mix_pass_shell(
+    Canvas& canvas,
+    const glm::vec2& bb_min,
+    const glm::vec2& bb_sz)
+{
+    return pp::panopainter::make_legacy_canvas_stroke_mix_pass_shell(
+        [&] {
+            canvas.m_mixer.bindFramebuffer();
+            canvas.apply_canvas_viewport(0, 0, canvas.m_mixer.getWidth(), canvas.m_mixer.getHeight());
+            canvas.apply_canvas_capability(canvas.depth_test_state(), false);
+            canvas.apply_canvas_capability(canvas.scissor_test_state(), true);
+            canvas.apply_canvas_capability(canvas.blend_state(), false);
+            canvas.apply_canvas_scissor(
+                static_cast<std::int32_t>(bb_min.x),
+                static_cast<std::int32_t>(bb_min.y),
+                static_cast<std::int32_t>(bb_sz.x),
+                static_cast<std::int32_t>(bb_sz.y));
+        },
+        [&] {
+            canvas.m_mixer.unbindFramebuffer();
+        });
+}
+
 glm::vec4 Canvas::stroke_draw_samples(
     int i,
     std::vector<vertex_t>& P,