Marketplace: item diff, manager, refresh and update detection

Adds diff::item_diff (similar), MarketplaceManager with snapshots,
persisted sync reports, the gh-login slot and a repo lock held across
fetches (pre-flight F11a), refresh_marketplace, load_cached_snapshot,
compute_updates, pins_by_marketplace, head_for, and the GitFixture test
helper on top of git::test_support (F3). AppState gains marketplace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src-tauri/src/lib.rs

@@ -49,6 +49,7 @@ pub struct AppState {
     /// preview is not actually binding on what gets applied.
     pub pending_settings_import:
         Arc<tokio::sync::Mutex<Option<commands::settings_export_commands::PendingSettingsImport>>>,
+    pub marketplace: Arc<marketplace::MarketplaceManager>,
 }
 
 // ─────────────────────────────────────────────────────────────────────────────
@@ -225,6 +226,13 @@ pub fn run() {
     let exec_manager = Arc::new(ExecSessionManager::new());
     let auth_bridge = Arc::new(AuthBridgeManager::new());
     let lifecycle = Arc::new(Lifecycle::new());
+    let marketplace = Arc::new(marketplace::MarketplaceManager::new(
+        dirs::data_dir()
+            .map(|d| d.join("triple-c"))
+            .unwrap_or_else(|| std::env::temp_dir().join("triple-c")),
+    ));
+    let marketplace_setup = marketplace.clone();
+    let _ = &marketplace_setup;
 
     // Clone Arcs for the setup closure (web terminal auto-start)
     let projects_store_setup = projects_store.clone();
@@ -243,6 +251,7 @@ pub fn run() {
             web_terminal_server: Arc::new(tokio::sync::Mutex::new(None)),
             lifecycle,
             pending_settings_import: Arc::new(tokio::sync::Mutex::new(None)),
+            marketplace,
         })
         .manage(file_viewer::registry::ViewerRegistry::default())
         .setup(move |app| {