Add stroke mix shell executor coverage

docs/modernization/debt.md

@@ -86,6 +86,9 @@ agent or engineer to remove them without reconstructing context from chat.
   now routes the remaining mixer framebuffer/capability shell through a local
   helper, leaving the call site with only mix-pass planning and helper
   invocation.
+- 2026-06-13: `LATER-003` was narrowed again. The retained stroke mix shell
+  now has direct executor regression coverage in
+  `retained_stroke_mix_pass_shell_builder_preserves_combined_wiring`.
 - 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

@@ -629,6 +629,9 @@ Progress Notes:
 - 2026-06-13: `Canvas::stroke_draw_mix()` now routes the remaining mixer
   framebuffer/capability shell through a local helper, leaving the call site
   with only mix-pass planning and helper invocation.
+- 2026-06-13: `retained_stroke_mix_pass_shell_builder_preserves_combined_wiring`
+  now also exercises the direct shell executor path, locking the combined mix
+  shell boundary with call-order regression coverage.
 - 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.

tests/paint_renderer/stroke_execution_tests.cpp

@@ -1959,6 +1959,15 @@ void retained_stroke_mix_pass_shell_builder_preserves_combined_wiring(pp::tests:
     shell.request.unbind_mix_samplers();
     shell.setup.end();
 
+    events.clear();
+    shell.setup.begin();
+    const auto shell_result = pp::panopainter::execute_legacy_canvas_stroke_mix_pass_shell(
+        shell.setup.begin,
+        shell.setup.end,
+        shell.request);
+    PP_EXPECT(h, shell_result.ok);
+    PP_EXPECT(h, shell_result.composed_planes == 1U);
+
     const std::vector<std::string> expected_events {
         "begin",
         "bind-samplers",