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/mod.rs

@@ -12,6 +12,7 @@ mod block_editor;
 #[cfg(test)]
 mod block_editor_tests;
 mod block_save_as;
+mod block_visibility;
 mod block_properties;
 #[cfg(test)]
 mod block_properties_tests;
@@ -589,6 +590,8 @@ pub(super) struct OpenCADStudio {
     block_definition_open_editor: Option<String>,
     block_edit_picker: Option<crate::ui::window::block_edit_picker::State>,
     block_save_as: Option<crate::ui::window::block_save_as::State>,
+    block_visibility: Option<crate::ui::window::block_visibility::State>,
+    block_visibility_mode: bool,
     /// PDF dialogs' working copies; `None` while closed.
     pdf_attach: Option<crate::ui::window::pdf_dialogs::PdfAttachState>,
     point_cloud_attach: Option<crate::ui::window::pdf_dialogs::PointCloudAttachState>,
@@ -2239,6 +2242,7 @@ pub enum ModalKind {
     BlockEditChanges,
     BlockEditPicker,
     BlockSaveAs,
+    BlockVisibility,
     SaveDialog,
     Recovery,
     /// Fonts referenced by the drawing are missing on this machine.
@@ -2884,6 +2888,7 @@ pub enum Message {
     BlockEditChangesCancel,
     BlockSaveAsName(String),
     BlockSaveAsApply,
+    BlockVisibility(crate::ui::window::block_visibility::Action),
     BlockEditPickerName(String),
     BlockEditPickerSelect(String),
     BlockEditPickerApply,
@@ -4658,6 +4663,8 @@ impl OpenCADStudio {
             block_definition_open_editor: None,
             block_edit_picker: None,
             block_save_as: None,
+            block_visibility: None,
+            block_visibility_mode: false,
             pdf_attach: None,
             point_cloud_attach: None,
             attdef_dialog: None,