feat(projects): project_manager module — DDL bootstrap, slug/paths, settings, reveal

Modulo nuevo que gestiona el sistema de proyectos del issue 0006.
Cada proyecto vive como subcarpeta junto al exe con su operations.db,
types.yaml y graph_explorer.db propios. Helpers:

- project_validate_slug / project_paths / project_list / project_exists
- project_create — bootstrap operations.db con DDL completo (entities,
  relations, fts5, triggers, assertions, executions, logs) + types.yaml
  semilla (copia de examples/types.yaml o embed si no existe).
- projects_migrate_legacy_layout — mueve operations.db / graph_explorer.db
  del cwd a projects/default/ si el directorio projects/ no existe.
- project_settings_load/save/touch — graph_explorer.ini con last_active
  y recent (max 5).
- project_reveal_in_explorer — Windows ShellExecute / Linux xdg-open.

CMakeLists registra project_manager.cpp en add_imgui_app.

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

CMakeLists.txt

@@ -21,6 +21,7 @@ add_imgui_app(graph_explorer
     types_registry.cpp
     layout_store.cpp
     entity_ops.cpp
+    project_manager.cpp
     # --- viz ---
     ${FN_CPP_ROOT_DIR}/functions/viz/graph_renderer.cpp
     ${FN_CPP_ROOT_DIR}/functions/viz/graph_force_layout.cpp