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