Extract fill modes, preview runtime pockets, and brush item UI

docs/modernization/roadmap.md

@@ -81,11 +81,11 @@ Current hotspot files:
 
 - `src/canvas.cpp`: 17 lines
 - `src/app_layout.cpp`: 125 lines
-- `src/canvas_modes.cpp`: 402 lines
+- `src/canvas_modes.cpp`: 176 lines
 - `src/node.cpp`: 260 lines
 - `src/main.cpp`: 130 lines
-- `src/node_panel_brush.cpp`: 231 lines
-- `src/node_stroke_preview.cpp`: 343 lines
+- `src/node_panel_brush.cpp`: 189 lines
+- `src/node_stroke_preview.cpp`: 280 lines
 - `src/node_canvas.cpp`: 219 lines
 - `src/app.cpp`: 113 lines
 - `src/app_dialogs.cpp`: 168 lines
@@ -309,6 +309,10 @@ Current architecture mismatches that must be treated as real blockers:
   `CanvasModeLine` interaction families now also route through
   `src/legacy_canvas_mode_pen_line.cpp` instead of staying inline in
   `src/canvas_modes.cpp`,
+  while the `CanvasModeFill` and `CanvasModeFloodFill` interaction families now
+  also route through `src/legacy_canvas_mode_fill.cpp` instead of staying
+  inline in `src/canvas_modes.cpp`, which materially thins another retained
+  fill-mode execution pocket from the broader canvas/render hotspot family,
   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
@@ -324,7 +328,13 @@ Current architecture mismatches that must be treated as real blockers:
   panel UI pocket, while the retained `LegacyBrushPresetListServices` block
   now also lives in `src/legacy_brush_preset_list_services.*` instead of
   staying inline in `src/node_panel_brush.cpp`, which trims another retained
-  preset popup tail now also lives in `src/legacy_brush_preset_panel_ui.*`
+  preset-list pocket, while `NodeButtonBrush` clone/init/icon/read/write/draw
+  behavior and `NodeBrushPresetItem` clone/init/draw behavior now also live in
+  `src/legacy_brush_panel_item_ui.*` instead of staying inline in
+  `src/node_panel_brush.cpp`, which trims the remaining brush-item UI pocket
+  from the live brush panel file, while `NodePanelBrushPreset` popup-close
+  event handling now also lives in
+  `src/legacy_brush_preset_panel_ui.*`
   instead of staying inline in `src/node_panel_brush.cpp`, which removes the
   last inline brush-panel popup close handler from the live node. The
   broader preset workflow pocket still remains, while `NodeCanvas::handle_event()`