Offer runnable C++ and Lua project starters

include/faset/editor/build_service.hpp

@@ -3,6 +3,7 @@
 #include <filesystem>
 #include <memory>
 #include <string>
+#include <string_view>
 #include <vector>
 
 namespace faset::editor {
@@ -38,6 +39,9 @@ class BuildService {
     BuildService(const BuildService&) = delete;
     BuildService& operator=(const BuildService&) = delete;
     void scaffold(const std::string& name, int dimension);
+    // Explicit starter choice creates a runnable start scene. The legacy overload above
+    // remains intentionally scene-free for existing command-line and API callers.
+    void scaffold(const std::string& name, int dimension, std::string_view language);
     std::string start_build();
     std::string start_cook(Json resolved_scene);
     // Publishes output/generations/<id>; current.json changes only after all validation succeeds.

include/faset/editor/project_launcher.hpp

@@ -8,6 +8,7 @@ struct ProjectSelection {
     std::string name;
     int dimension = 3;
     bool create = false;
+    std::string language = "cpp";
 };
 // Records only existing, valid projects. Call after successful Session setup.
 void remember_project(const std::filesystem::path& project);
@@ -15,7 +16,9 @@ class ProjectLauncher {
   public:
     ProjectLauncher(render::Renderer&, const std::filesystem::path& engine_root,
                     const std::filesystem::path& initial_project = {},
-                    const std::filesystem::path& recent_store = {});
+                    const std::filesystem::path& recent_store = {},
+                    const std::optional<ProjectSelection>& retry = {},
+                    std::string creation_error = {});
     ~ProjectLauncher();
     void frame(const std::vector<render::Event>&);
     const render::Snapshot& snapshot() const;
@@ -30,5 +33,7 @@ class ProjectLauncher {
 std::optional<ProjectSelection>
 run_project_launcher(const std::filesystem::path& engine_root,
                      const std::filesystem::path& initial_project = {},
-                     std::uint64_t max_frames = 0, const std::filesystem::path& capture = {});
+                     std::uint64_t max_frames = 0, const std::filesystem::path& capture = {},
+                     const std::optional<ProjectSelection>& retry = {},
+                     std::string creation_error = {});
 } // namespace faset::editor

include/faset/editor/session.hpp

@@ -6,6 +6,7 @@
 #include <faset/editor/plugins.hpp>
 #include <memory>
 #include <mutex>
+#include <string_view>
 #include <thread>
 
 namespace faset::editor {
@@ -33,6 +34,7 @@ class Session {
         return plugins_ ? plugins_->panels() : Json::array();
     }
     void scaffold(const std::string& name, int dimension);
+    void scaffold(const std::string& name, int dimension, std::string_view language);
     const SessionConfig& config() const {
         return config_;
     }