Extract draw merge temporary composite wrapper

docs/modernization/debt.md

@@ -76,6 +76,10 @@ agent or engineer to remove them without reconstructing context from chat.
   `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,
   temporary-stroke branch selection, and concrete layer RTT ownership remain in
   the legacy Canvas path.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
+  composite setup now routes through `execute_canvas_draw_merge_temporary_composite(...)`;
+  setup, sampler, texture, draw, and unbind callbacks still remain retained in
+  the legacy Canvas path.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes retained preview feedback/material/composite planning plus stroke
   shader uniform assembly through

docs/modernization/tasks.md

@@ -655,6 +655,11 @@ Progress Notes:
   `Canvas`. Next slice should target another narrow draw-merge execution seam
   without reopening landed temporary-composite, layer-blend, final-plane, or
   texture-alpha helpers.
+- 2026-06-13: `Canvas::draw_merge()` temporary composite setup now routes
+  through `execute_canvas_draw_merge_temporary_composite(...)`; the retained
+  path still owns the concrete setup, sampler, texture, draw, and unbind
+  callbacks. Next slice should target another narrow draw-merge seam without
+  reopening the landed temporary-composite helper.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained frame-plan assembly for previous-sample projection mode and zoom
   scaling. Next slice should target the remaining preview/Canvas stroke

src/canvas.cpp

@@ -480,6 +480,22 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
     gl.restore();
 }
 
+template <typename Setup, typename BindSamplers, typename BindTextures, typename Draw, typename UnbindTextures>
+static void execute_canvas_draw_merge_temporary_composite(
+    Setup&& setup,
+    BindSamplers&& bind_samplers,
+    BindTextures&& bind_textures,
+    Draw&& draw,
+    UnbindTextures&& unbind_textures)
+{
+    pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
+        std::forward<Setup>(setup),
+        std::forward<BindSamplers>(bind_samplers),
+        std::forward<BindTextures>(bind_textures),
+        std::forward<Draw>(draw),
+        std::forward<UnbindTextures>(unbind_textures));
+}
+
 std::array<std::vector<vertex_t>, 6> Canvas::stroke_draw_project(std::array<vertex_t, 4>& B, bool project_3d /*= false*/, glm::mat4 mv /*= glm::mat4(1)*/) const
 {
     // intersect P with the current face to clip diverging points from the plane
@@ -1435,7 +1451,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 use_blend,
                 pp::panopainter::LegacyCanvasDrawMergeLayerCompositeExecution {
                     .execute_temporary_erase = [&] {
-                        pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
+                        execute_canvas_draw_merge_temporary_composite(
                     [&] {
                         //ShaderManager::u_vec2(kShaderUniform::Resolution, zw(m_box) / zoom);
                         //ShaderManager::u_int(kShaderUniform::Lock, m_layers[layer_index]->m_alpha_locked);
@@ -1479,7 +1495,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                         if (b->m_pattern_flipx) patt_scale.x *= -1.f;
                         if (b->m_pattern_flipy) patt_scale.y *= -1.f;
 
-                        pp::panopainter::execute_legacy_canvas_stroke_temporary_composite(
+                        execute_canvas_draw_merge_temporary_composite(
                     [&] {
                         pp::panopainter::setup_legacy_stroke_composite_shader(
                             pp::panopainter::LegacyStrokeCompositeUniforms {