Trim stroke mix callsite

docs/modernization/debt.md

@@ -97,6 +97,8 @@ agent or engineer to remove them without reconstructing context from chat.
 - 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: `LATER-003` was narrowed again. `Canvas::stroke_draw_mix()`
+  now keeps only retained shell assembly and executor dispatch at the callsite.
 - 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

@@ -641,6 +641,9 @@ Progress Notes:
 - 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_mix()` now keeps just the retained shell
+  assembly and executor dispatch at the callsite; the framebuffer setup
+  callbacks are isolated in the helper.
 - 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

@@ -390,8 +390,6 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
 {
     gl_state gl;
     gl.save();
-    const auto layer_index = m_current_layer_idx;
-    auto& current_layer = *m_layers[layer_index];
     const auto mix_shell = make_canvas_stroke_mix_pass_shell(*this, bb_min, bb_sz);
     [[maybe_unused]] const auto mix_result = pp::panopainter::execute_legacy_canvas_stroke_mix_pass_shell(
         mix_shell.setup.begin,