test: integration-test the mode persist-on-unload wiring (#14)

The post-/runda DA pass on 4cd574b found the persist-on-unload wiring
(quit + project switch calling Database::set_mode) had no integration
test — only the db-level set_mode behaviour was covered, not that the
runtime actually invokes it on unload.

Add runtime::switch_persists_the_outgoing_projects_mode, driving the
real handle_project_switch end-to-end and asserting the outgoing
project's project.yaml recorded the mode it was left in. Red-first
verified: with the set_mode call disabled it fails (None vs
Some(Advanced)). The quit unload site shares the same set_mode call;
Action::Quit emission is already covered in app tests.

Updates ADR-0015 Amendment 1 coverage note.

docs/adr/0015-project-storage-runtime.md

@@ -677,9 +677,13 @@ time").
   (the teacher-export round-trip); `cli` `--mode` parse/precedence;
   `app::{mode_command_changes_mode_and_emits_persist_action,
   mode_command_via_one_shot_escape_persists_advanced,
-  project_switched_event_restores_the_stored_mode}`. The runtime's
+  project_switched_event_restores_the_stored_mode}`;
-  unload call sites (quit + `handle_project_switch`) are thin
+  `runtime::switch_persists_the_outgoing_projects_mode` — an
-  wiring over the tested `Database::set_mode`.
+  integration test driving the real `handle_project_switch` to
+  prove the unload wiring calls `set_mode` (red-first verified).
+  The quit unload site shares that `set_mode` call; the
+  `Action::Quit` emission is covered by `app`'s
+  `quit_command_returns_quit_action` / `ctrl_c_returns_quit_action`.
 
 ### Relationship to the Iteration 6 backlog
 

src/runtime.rs

@@ -3434,4 +3434,59 @@ mod tests {
             "single-line FK echo when the child column already existed",
         );
     }
+
+    #[tokio::test]
+    async fn switch_persists_the_outgoing_projects_mode() {
+        // ADR-0015 mode-restore amendment (issue #14), persist on
+        // unload — exercised through the real `handle_project_switch`
+        // path, not just the db-level `set_mode`. Proves the wiring
+        // calls `set_mode` on the outgoing project before it is
+        // dropped. The outgoing project is *named* so it survives the
+        // switch (an unmodified temp would be cleaned up, taking its
+        // project.yaml with it). Without the unload persist the
+        // outgoing skeleton carries no `mode:` → `None`.
+        use super::{handle_project_switch, Session, SwitchRequest};
+        use crate::db::Database;
+        use crate::mode::Mode;
+        use crate::persistence::Persistence;
+        use crate::project::{projects_dir, Project};
+        use tokio::sync::mpsc;
+
+        let data_root = tempfile::tempdir().unwrap();
+        let projects = projects_dir(data_root.path());
+        std::fs::create_dir_all(&projects).unwrap();
+        let outgoing_path = projects.join("Outgoing");
+        let outgoing = Project::create_named(&outgoing_path).unwrap();
+        let db_path = outgoing.db_path();
+        let persistence =
+            Persistence::new(outgoing.path().to_path_buf()).with_mode(Mode::Advanced);
+        let database =
+            Database::open_with_persistence_and_undo(&db_path, persistence, true).unwrap();
+        let mut session = Session {
+            project: Some(outgoing),
+            database: Some(database),
+            data_root: data_root.path().to_path_buf(),
+        };
+        // Keep the receiver alive so the switch's events aren't sent
+        // into a closed channel.
+        let (tx, _rx) = mpsc::channel(64);
+
+        // Switch away to a fresh temp; the App's live mode here is
+        // Advanced (the outgoing project's mode).
+        handle_project_switch(
+            &mut session,
+            SwitchRequest::NewTemp,
+            "new".to_string(),
+            &tx,
+            true,
+            Mode::Advanced,
+        )
+        .await;
+
+        assert_eq!(
+            Persistence::read_stored_mode(&outgoing_path),
+            Some(Mode::Advanced),
+            "switching away must persist the outgoing project's mode",
+        );
+    }
 }