Extract stroke mix shell helper

docs/modernization/debt.md

@@ -82,6 +82,10 @@ agent or engineer to remove them without reconstructing context from chat.
   now calls the retained mix-pass executor directly instead of a local wrapper
   shell helper; the live path still owns the concrete mixer framebuffer setup
   and GL capability toggles.
+- 2026-06-13: `LATER-003` was narrowed again. `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: `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

@@ -626,6 +626,9 @@ Progress Notes:
 - 2026-06-13: `Canvas::stroke_draw_mix()` now calls the retained mix-pass
   executor directly instead of a local wrapper shell helper; the live path
   still owns the concrete mixer framebuffer setup and GL capability toggles.
+- 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: `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.