Extract stroke mix shell wrapper

docs/modernization/debt.md

@@ -441,6 +441,10 @@ agent or engineer to remove them without reconstructing context from chat.
   preserving caller-specific texture slot uniforms. Mixer framebuffer,
   viewport/scissor/capability state, sampler binding, texture binding, and draw
   execution remain retained Canvas code.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_mix()` now
+  routes retained mix-shell execution through a local wrapper around
+  `execute_legacy_canvas_stroke_mix_pass_shell(...)`; the live path still owns
+  the concrete mixer framebuffer setup and GL capability toggles.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` checkerboard
   background shader setup and final merged-texture redraw setup now route
   through `legacy_canvas_draw_merge_services.h`. The retained Canvas path still

docs/modernization/tasks.md

@@ -619,6 +619,10 @@ Progress Notes:
   closeout.
 - 2026-06-13: `STR-004` closed the last inline stroke dispatch glue. `LATER-003`
   remains at the binding-only tail.
+- 2026-06-13: `Canvas::stroke_draw_mix()` now routes the retained shell
+  execution through a local wrapper around
+  `execute_legacy_canvas_stroke_mix_pass_shell(...)`; the live path still owns
+  the concrete mixer framebuffer setup and GL capability toggles.
 - 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.