feat(select): Previous / Last keywords at Select objects prompts

Standard selection keywords, available typed or as prompt buttons in
every gathering command (#426): P / PREVIOUS re-selects the set the
last finishing command worked on (now captured before the working set
is dropped), and L / LAST selects the most recently created object of
the current space (highest handle among its selectable wires). Handled
centrally in the step router, so the command line, the option buttons
and the automation feeder share one implementation; deleted objects
and locked layers are filtered out.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/document.rs

@@ -160,6 +160,10 @@ pub(super) struct DocumentTab {
     pub(super) properties: PropertiesPanel,
     pub(super) layers: LayerPanel,
     pub(super) active_cmd: Option<Box<dyn CadCommand>>,
+    /// The selection set the most recent command worked on, captured when a
+    /// finishing command drops the live selection — re-selectable with the
+    /// "Previous" keyword at any Select objects prompt (#426).
+    pub(super) prev_selection: Vec<acadrust::Handle>,
     pub(super) last_cmd: Option<String>,
     pub(super) snap_result: Option<SnapResult>,
     pub(super) active_grip: Option<GripEdit>,
@@ -411,6 +415,7 @@ impl DocumentTab {
             scene,
             current_path: None,
             dirty: false,
+            prev_selection: Vec::new(),
             tab_title: format!("Drawing{}", n),
             properties: PropertiesPanel::empty(),
             layers: LayerPanel::default(),