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/app/view/mod.rs

@@ -2218,6 +2218,7 @@ bg={bg_ms:.1}ms n={view_count}"
                     self.tabs[self.active_tab].history.undo_stack.len(),
                     self.tabs[self.active_tab].history.redo_stack.len(),
                     self.show_block_palette,
+                    self.block_visibility_context(),
                 ));
             }
             // Split the `chrome` bucket so live `view-detail` traces show

src/app/view/modal.rs

@@ -86,6 +86,7 @@ impl OpenCADStudio {
             Some(K::Unsaved) => crate::tr!("modal", "unsaved-changes"),
             Some(K::BlockEditChanges) => crate::t!("Block Editor").into_owned(),
             Some(K::BlockEditPicker) => crate::t!("Edit Block Definition").into_owned(),
+            Some(K::BlockVisibility) => crate::t!("Visibility States").into_owned(),
             Some(K::BlockSaveAs) => crate::t!("Save Block As").into_owned(),
             Some(K::PointStyle) => crate::tr!("modal", "point-style"),
             Some(K::AttributeEditor) => crate::tr!("modal", "attribute-editor"),
@@ -520,6 +521,12 @@ impl OpenCADStudio {
                     crate::ui::window::block_edit_picker::view(state, flow)
                 })
             }
+            super::super::ModalKind::BlockVisibility => {
+                let state = self.block_visibility.as_ref()?;
+                sized_flow(ex, 580, 470, |flow| {
+                    crate::ui::window::block_visibility::view(state, flow)
+                })
+            }
             super::super::ModalKind::BlockSaveAs => {
                 let state = self.block_save_as.as_ref()?;
                 sized_flow(ex, 430, 370, |flow| {