Browser and viewport: item menu, per-item eyes, Delete key with confirmation

Right-click on any browser item (body, surface, datum, partition, set,
material, section, step, boundary condition, load, reference point,
coupling, contact, fluid domain) opens its menu: Edit…, Hide/Show,
Isolate, Show All, Active in step (boundary conditions and loads) and
Delete…. A right click on a datum or a partition's cut face in the
viewport opens the same menu. Each item the viewport draws on its own
has an eye; hidden items are neither drawn nor picked. Datums and
partitions are selectable with properties. Delete… and the Delete key
ask first (the prompt lists what uses the item: a datum's partitions
are deleted with it, other definitions in use are refused); ui.prompt
answers it. Every entry runs the same command as the ribbon and forms.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

crates/engine/src/lib.rs

@@ -18,6 +18,7 @@ pub mod cmd;
 
 pub use cmd::analytic_surfaces;
 pub use cmd::replace::{MESHERS, Replacement, replacement};
+pub use fieldcraft_io::{Item as ViewItem, ItemKind};
 pub mod history;
 pub mod progress;
 pub mod requirement;