feat: index runtime sessions by pane

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

@@ -404,13 +404,27 @@ impl ShellModel {
     ///
     /// Returns [`InvalidExternalOutputPane`] when `pane` is not a terminal.
     pub fn with_external_output(pane: PaneId) -> Result<Self, InvalidExternalOutputPane> {
+        Self::with_external_outputs([pane])
+    }
+
+    /// Creates the comparison model with actor-driven output for each named
+    /// terminal while non-terminal surfaces retain deterministic updates.
+    ///
+    /// # Errors
+    ///
+    /// Returns [`InvalidExternalOutputPane`] if any pane is not a terminal.
+    pub fn with_external_outputs(
+        panes: impl IntoIterator<Item = PaneId>,
+    ) -> Result<Self, InvalidExternalOutputPane> {
         let mut model = Self::default();
-        let pane_state = &mut model.panes[pane.index()];
-        if pane_state.kind() != SurfaceKind::Terminal {
-            return Err(InvalidExternalOutputPane);
+        for pane in panes {
+            let pane_state = &mut model.panes[pane.index()];
+            if pane_state.kind() != SurfaceKind::Terminal {
+                return Err(InvalidExternalOutputPane);
+            }
+            pane_state.output_source = OutputSource::External;
+            pane_state.lines.clear();
         }
-        pane_state.output_source = OutputSource::External;
-        pane_state.lines.clear();
         Ok(model)
     }
 
@@ -990,6 +1004,32 @@ mod tests {
         assert_eq!(model.counters().external_snapshot_updates, 1);
     }
 
+    #[test]
+    fn three_external_terminals_keep_independent_output_and_three_fixtures() {
+        let mut model = ShellModel::with_external_outputs([
+            PaneId::CodexRuntime,
+            PaneId::ClaudeUi,
+            PaneId::PiDocs,
+        ])
+        .unwrap();
+        let tick = model.dispatch(ShellAction::SyntheticStreamTick);
+        assert_eq!(tick.terminal_lines_appended, 0);
+        assert_eq!(model.counters().surface_updates, 3);
+
+        model.dispatch(ShellAction::ReplaceExternalOutput {
+            pane: PaneId::ClaudeUi,
+            lines: vec!["claude shell".into()],
+        });
+        model.dispatch(ShellAction::ReplaceExternalOutput {
+            pane: PaneId::PiDocs,
+            lines: vec!["pi shell".into()],
+        });
+        assert_eq!(model.pane(PaneId::ClaudeUi).lines(), ["claude shell"]);
+        assert_eq!(model.pane(PaneId::PiDocs).lines(), ["pi shell"]);
+        assert!(model.pane(PaneId::CodexRuntime).lines().is_empty());
+        assert_eq!(model.counters().external_snapshot_updates, 2);
+    }
+
     #[test]
     fn external_output_rejects_nonterminals_and_deterministic_panes() {
         assert!(ShellModel::with_external_output(PaneId::Architecture).is_err());