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.