feat(projects): integrate project switcher in app shell + Inspector menu

main.cpp:
- Forward decl + switch_to_project: cierra layout_store, libera grafo,
  aplica nuevos paths, vuelve a cargar.
- apply_project_paths: deriva operations.db/types.yaml/graph_explorer.db
  del slug y los expone a g_app.active_project.
- main: arg --project <slug>; modo legacy si --input/positional dado;
  modo proyecto si no — migra layout legacy, decide target via
  arg/last_active/'default', crea si no existe, abre BDs y carga.
- render(): handler want_switch_project + monta views_new_project_modal.

views.h: AppState gana active_project, want_switch_project,
switch_project_target, show_new_project_modal, new_project_buf,
new_project_error, project_list_cache, project_recent_cache.

views.cpp:
- Toolbar: boton 'Project: <slug>' con popup (New/Recent/Open/Reveal).
  Refresca caches al abrir el menu.
- views_new_project_modal: input slug + validacion + creacion + switch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

views.h

@@ -52,6 +52,16 @@ struct AppState {
     bool                 want_open_file   = false;  // marcado al confirmar el modal Open
     char                 open_buf[512]    = {};
 
+    // Project system (issue 0006)
+    std::string          active_project;            // slug del proyecto activo
+    bool                 want_switch_project = false;
+    std::string          switch_project_target;     // slug objetivo del switch
+    bool                 show_new_project_modal = false;
+    char                 new_project_buf[80] = {};
+    std::string          new_project_error;         // mensaje a mostrar en el modal
+    std::vector<std::string> project_list_cache;    // refrescado al abrir el menu Project
+    std::vector<std::string> project_recent_cache;
+
     // Labels overlay
     bool                 labels_enabled   = true;
 
@@ -108,6 +118,10 @@ bool views_filters_modal(AppState& app);
 // Modal Open file — text input + boton Open.
 bool views_open_modal(AppState& app);
 
+// Modal "New project..." — slug input con validacion. Devuelve true si el
+// usuario confirma con un slug valido.
+bool views_new_project_modal(AppState& app);
+
 // Refresca los flags `flags` de cada nodo/arista segun el array
 // `type_visible[]` / `rel_type_visible[]`. Lineal en N+M.
 void views_apply_visibility(AppState& app);