feat(session-server): add session server for app launching, remove apps from init manifest

Init now only starts servers. The session server holds ProcessCreate cap
and launches apps on demand. Snake is no longer in the boot manifest.

crates/servers/session-server/src/tests/server.rs

@@ -0,0 +1,78 @@
+use crate::protocol::*;
+use crate::server::SessionServer;
+use server_framework::Server;
+
+fn test_server() -> SessionServer {
+    let mut s = SessionServer::new(0, 0, 0);
+    s.register_app("snake", "/apps/snake.elf");
+    s.register_app("clock", "/apps/clock.elf");
+    s
+}
+
+#[test]
+fn list_apps_returns_count() {
+    let mut s = test_server();
+    let resp = s.handle(SessionRequest::ListApps);
+    assert_eq!(resp, SessionResponse::AppList { count: 2 });
+}
+
+#[test]
+fn launch_valid_app() {
+    let mut s = test_server();
+    let resp = s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    assert!(matches!(resp, SessionResponse::Launched { .. }));
+}
+
+#[test]
+fn launch_sets_active() {
+    let mut s = test_server();
+    s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    let resp = s.handle(SessionRequest::ActiveApp);
+    assert_eq!(resp, SessionResponse::Active { app: AppId::new(0) });
+}
+
+#[test]
+fn launch_invalid_app_returns_not_found() {
+    let mut s = test_server();
+    let resp = s.handle(SessionRequest::Launch { app: AppId::new(99) });
+    assert_eq!(resp, SessionResponse::NotFound);
+}
+
+#[test]
+fn launch_already_running_returns_error() {
+    let mut s = test_server();
+    s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    let resp = s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    assert_eq!(resp, SessionResponse::Error);
+}
+
+#[test]
+fn kill_running_app() {
+    let mut s = test_server();
+    s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    let resp = s.handle(SessionRequest::Kill { app: AppId::new(0) });
+    assert_eq!(resp, SessionResponse::Killed);
+}
+
+#[test]
+fn kill_clears_active() {
+    let mut s = test_server();
+    s.handle(SessionRequest::Launch { app: AppId::new(0) });
+    s.handle(SessionRequest::Kill { app: AppId::new(0) });
+    let resp = s.handle(SessionRequest::ActiveApp);
+    assert_eq!(resp, SessionResponse::NotFound);
+}
+
+#[test]
+fn kill_not_running_returns_not_found() {
+    let mut s = test_server();
+    let resp = s.handle(SessionRequest::Kill { app: AppId::new(0) });
+    assert_eq!(resp, SessionResponse::NotFound);
+}
+
+#[test]
+fn no_active_app_initially() {
+    let mut s = test_server();
+    let resp = s.handle(SessionRequest::ActiveApp);
+    assert_eq!(resp, SessionResponse::NotFound);
+}