Remove unused stroke mix planning

docs/modernization/debt.md

@@ -92,6 +92,8 @@ agent or engineer to remove them without reconstructing context from chat.
 - 2026-06-13: `LATER-003` was narrowed again. The retained stroke mix shell
   now also has isolated executor regression coverage in
   `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: `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

@@ -635,6 +635,9 @@ Progress Notes:
 - 2026-06-13: `retained_stroke_mix_pass_shell_executor_preserves_combined_wiring`
   now covers the direct shell executor path separately, keeping the shell
   boundary regression isolated from the builder coverage.
+- 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_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

@@ -394,14 +394,6 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
     auto& current_layer = *m_layers[layer_index];
     std::array<glm::mat4, 6> plane_transform {};
     std::copy(std::begin(m_plane_transform), std::end(m_plane_transform), plane_transform.begin());
-    const auto mix_planes = pp::panopainter::plan_legacy_canvas_stroke_mix_pass_planes(
-        current_layer.m_visible,
-        current_layer.m_opacity,
-        glm::scale(glm::vec3(1, -1, 1)) * m_proj * m_mv,
-        plane_transform,
-        [&](int plane_index) {
-            return current_layer.face(plane_index);
-        });
     const auto mix_shell = pp::panopainter::make_legacy_canvas_stroke_mix_pass_shell(
         [&] {
             m_mixer.bindFramebuffer();
@@ -422,7 +414,6 @@ void Canvas::stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz)
         mix_shell.setup.begin,
         mix_shell.setup.end,
         mix_shell.request);
-    (void)mix_planes;
 
     gl.restore();
 }