Extract canvas layer flows, pen modes, and node attributes

docs/modernization/roadmap.md

@@ -79,10 +79,10 @@ What is still carrying too much live ownership:
 
 Current hotspot files:
 
-- `src/canvas.cpp`: 1490 lines
+- `src/canvas.cpp`: 1271 lines
 - `src/app_layout.cpp`: 125 lines
-- `src/canvas_modes.cpp`: 1014 lines
-- `src/node.cpp`: 995 lines
+- `src/canvas_modes.cpp`: 720 lines
+- `src/node.cpp`: 803 lines
 - `src/main.cpp`: 271 lines
 - `src/node_panel_brush.cpp`: 435 lines
 - `src/node_stroke_preview.cpp`: 751 lines
@@ -264,11 +264,18 @@ Current architecture mismatches that must be treated as real blockers:
   `Canvas::stroke_draw()` orchestration now also routes through
   `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, while the
+  stroke-render pocket, while `Canvas::layer_merge(...)`,
+  `Canvas::flood_fill(...)`, and `Canvas::FloodData::apply()` now also route
+  through `src/legacy_canvas_layer_services.cpp` instead of staying inline in
+  `src/canvas.cpp`, which trims another coherent retained layer/fill workflow
+  pocket, while the
   `CanvasModeTransform` interaction family now also routes through
   `src/legacy_canvas_mode_transform.cpp` instead of staying inline in
   `src/canvas_modes.cpp`, which materially thins another retained canvas-view
-  and transform-mode execution pocket,
+  and transform-mode execution pocket, while the `CanvasModePen` and
+  `CanvasModeLine` interaction families now also route through
+  `src/legacy_canvas_mode_pen_line.cpp` instead of staying inline in
+  `src/canvas_modes.cpp`,
   while `NodePanelBrush` save/restore/scan/reload/find/get-path ownership now
   routes through `src/legacy_brush_panel_services.*` instead of staying inline
   in `src/node_panel_brush.cpp`, which trims another retained brush-workflow
@@ -306,7 +313,9 @@ Current architecture mismatches that must be treated as real blockers:
   about without yet moving ownership into `pp_ui_core`, while the generic
   per-frame node execution/traversal family for `restore_context`,
   `clear_context`, `update`, `update_internal`, and `tick` now also lives in
-  `src/legacy_ui_node_execution.cpp`,
+  `src/legacy_ui_node_execution.cpp`, while `Node::parse_attributes(...)` now
+  also routes through `src/legacy_ui_node_attributes.*` instead of staying
+  inline in `src/node.cpp`,
   while `Canvas` point-trace/unproject/project/camera push-pop-get-set and
   face-to-shape helpers now also route through
   `src/legacy_canvas_projection_services.*` instead of staying inline in