Extract canvas layer flows, pen modes, and node attributes

docs/modernization/tasks.md

@@ -91,7 +91,7 @@ Status: In Progress
 
 Why now:
 `src/canvas.cpp` is still the biggest single architectural blocker at about
-1490 lines, with `src/canvas_modes.cpp` still large at about 1014 lines.
+1271 lines, with `src/canvas_modes.cpp` still large at about 720 lines.
 
 Current slice:
 - Canvas state-management helpers for picking, clear/clear-all, layer
@@ -130,6 +130,15 @@ Current slice:
   `src/legacy_canvas_stroke_live_services.cpp` instead of staying inline in
   `src/canvas.cpp`, which materially thins another large retained live
   stroke-render pocket from the canvas shell.
+- `Canvas::layer_merge(...)`, `Canvas::flood_fill(...)`, and
+  `Canvas::FloodData::apply()` now also live in
+  `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.
+- 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
+  interaction pocket from the broader canvas/render hotspot family.
 
 Write scope:
 - `src/canvas.cpp`
@@ -728,6 +737,10 @@ Current slice:
   `src/legacy_ui_node_event.*` instead of staying inline in `src/node.cpp`,
   which materially thins the base scene-graph event shell without changing its
   public surface.
+- `Node::parse_attributes(...)` now also routes through
+  `src/legacy_ui_node_attributes.*` instead of staying inline in `src/node.cpp`,
+  which trims another coherent XML/Yoga attribute decoding pocket from the base
+  scene-graph shell without changing its public surface.
 
 Write scope:
 - `src/node.cpp`