feat: add interactive VT and workspace command plane

spikes/ui-shell-model/src/lib.rs

@@ -288,6 +288,7 @@ pub enum ShellAction {
     SetCommandPaletteQuery(String),
     SyntheticStreamTick,
     AppendExternalOutput { pane: PaneId, lines: Vec<String> },
+    ReplaceExternalOutput { pane: PaneId, lines: Vec<String> },
 }
 
 /// Counters prove both candidates replayed the same workload. Framework
@@ -308,6 +309,7 @@ pub struct MeasurementCounters {
     pub terminal_lines_appended: u64,
     pub external_output_batches: u64,
     pub external_lines_appended: u64,
+    pub external_snapshot_updates: u64,
 }
 
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
@@ -536,6 +538,21 @@ impl ShellModel {
                     changed = true;
                 }
             }
+            ShellAction::ReplaceExternalOutput { pane, mut lines } => {
+                let pane = &mut self.panes[pane.index()];
+                if pane.output_source == OutputSource::External {
+                    if lines.len() > self.terminal_line_limit {
+                        lines.drain(..lines.len() - self.terminal_line_limit);
+                    }
+                    if pane.lines != lines {
+                        pane.lines = lines;
+                        self.counters.surface_updates += 1;
+                        self.counters.external_output_batches += 1;
+                        self.counters.external_snapshot_updates += 1;
+                        changed = true;
+                    }
+                }
+            }
         }
         if changed {
             self.revision += 1;
@@ -786,6 +803,14 @@ mod tests {
         assert_eq!(model.counters().surface_updates, 6);
         assert_eq!(model.counters().external_output_batches, 1);
         assert_eq!(model.counters().external_lines_appended, 2);
+
+        let snapshot = model.dispatch(ShellAction::ReplaceExternalOutput {
+            pane: PaneId::CodexRuntime,
+            lines: vec!["prompt $".into()],
+        });
+        assert!(snapshot.changed);
+        assert_eq!(model.pane(PaneId::CodexRuntime).lines(), ["prompt $"]);
+        assert_eq!(model.counters().external_snapshot_updates, 1);
     }
 
     #[test]