Extract canvas stroke runtime and preview draw shells

docs/modernization/tasks.md

@@ -91,7 +91,8 @@ Status: In Progress
 
 Why now:
 `src/canvas.cpp` is still the biggest single architectural blocker at about
-1271 lines, with `src/canvas_modes.cpp` still large at about 720 lines.
+1010 lines, with `src/canvas_modes.cpp` now materially thinner and the next
+remaining render-shell pressure shifting toward preview/canvas nodes.
 
 Current slice:
 - Canvas state-management helpers for picking, clear/clear-all, layer
@@ -135,6 +136,12 @@ Current slice:
   `src/legacy_canvas_layer_services.cpp` instead of staying inline in
   `src/canvas.cpp`, which trims another coherent retained layer/fill workflow
   pocket from the live canvas shell.
+- `Canvas::stroke_end(...)`, `Canvas::stroke_cancel(...)`,
+  `Canvas::stroke_draw_mix(...)`, `Canvas::stroke_draw_project(...)`,
+  `Canvas::stroke_update(...)`, and `Canvas::stroke_start(...)` now also live
+  in `src/legacy_canvas_stroke_runtime_services.*` instead of staying inline in
+  `src/canvas.cpp`, which trims another large retained live stroke/runtime
+  pocket from the canvas shell.
 - The `CanvasModePen` and `CanvasModeLine` interaction families now also live
   in `src/legacy_canvas_mode_pen_line.cpp` instead of staying inline in
   `src/canvas_modes.cpp`, which materially thins another retained pen/line
@@ -307,6 +314,16 @@ Current slice:
   live in `src/legacy_node_canvas_state_services.*` instead of staying inline
   in `src/node_canvas.cpp`, which materially thins another retained
   state/control pocket without reopening the draw path.
+- `NodeStrokePreview` retained lifecycle, worker-thread shell, render-to-image,
+  on-screen handling, and preview texture ownership now also live in
+  `src/legacy_node_stroke_preview_runtime_services.cpp` instead of staying
+  inline in `src/node_stroke_preview.cpp`, which materially thins the preview
+  node around its runtime-facing shell even though live pass execution still
+  remains.
+- `NodeCanvas::init()` plus the remaining `NodeCanvas::draw()` outer shell now
+  also live in `src/legacy_node_canvas_draw_services.*` instead of staying
+  inline in `src/node_canvas.cpp`, which materially reduces the live node to a
+  thinner controller surface around event routing and state wrappers.
 
 Write scope:
 - `src/node_stroke_preview.cpp`