feat(block-editor): add functional Visibility ribbon controls

Add the current-state selector, Visibility States dialog and mode/show/hide
controls to the contextual ribbon, using shared icons, tooltips, modal UI
and adaptive flyouts. Disable the controls until a Visibility parameter exists.

Stage state creation, rename, deletion, current-state changes and reordering
until OK. Support copied, show-all and hide-all new states in one undo step,
preserve native graph metadata, and restore current-state identity on undo/redo.

Implement nonsaved BVMODE and display-only reveal during BVSHOW selection.
Cancelling selection leaves saved visibility flags and history unchanged.
Keep the existing command-line state workflows available.

Validation: 6 targeted tests; 277 block, 31 app dock, 35 ribbon and 12 history
checks passed (overlapping filters). All 17 local DWG fixtures passed in-memory
DWG/DXF preservation checks with unchanged original checksums.

src/modules/block_editor/mod.rs

@@ -94,6 +94,41 @@ impl CadModule for BlockEditorModule {
                     ),
                 );
             }
+            groups.push(RibbonGroup {
+                title: "Visibility",
+                tools: vec![
+                    RibbonItem::LargeTool(ToolDef {
+                        id: "BVSTATE_MANAGER",
+                        label: "Visibility States",
+                        icon: IconKind::Svg(include_bytes!("../../../assets/icons/ui/list.svg")),
+                        event: ModuleEvent::Command("BVSTATE DIALOG".into()),
+                    }),
+                    RibbonItem::Tool(ToolDef {
+                        id: "BVMODE_TOGGLE",
+                        label: "Visibility Mode",
+                        icon: IconKind::Svg(include_bytes!(
+                            "../../../assets/icons/constrain/show_all.svg"
+                        )),
+                        event: ModuleEvent::Command("BVMODE TOGGLE".into()),
+                    }),
+                    RibbonItem::Tool(ToolDef {
+                        id: "BVSHOW",
+                        label: "Make Visible",
+                        icon: IconKind::Svg(include_bytes!(
+                            "../../../assets/icons/constrain/show.svg"
+                        )),
+                        event: ModuleEvent::Command("BVSHOW".into()),
+                    }),
+                    RibbonItem::Tool(ToolDef {
+                        id: "BVHIDE",
+                        label: "Make Invisible",
+                        icon: IconKind::Svg(include_bytes!(
+                            "../../../assets/icons/constrain/hide_all.svg"
+                        )),
+                        event: ModuleEvent::Command("BVHIDE".into()),
+                    }),
+                ],
+            });
             groups
         })
     }