feat(primitives_gallery): wire text_editor + file_watcher demo

- demos_text_editor.cpp: split horizontal con editor GLSL precargado a la
  izquierda (boton Save to /tmp/fn_demo.glsl + dirty indicator) y panel de
  eventos a la derecha (path, active flag, lista scrollable, boton clear).
  Watcher activo sobre /tmp/fn_demo.glsl; reintenta el add() tras el primer
  Save si el archivo no existia al iniciar.
- demos.h: declaracion de gallery::demo_text_editor()
- main.cpp: entry "text_editor"/"text_editor + watcher" en categoria Core
- CMakeLists.txt: anade demos_text_editor.cpp + sources de text_editor,
  file_watcher y vendor TextEditor.cpp + include path de imgui_text_edit

Nota: la primitives_gallery NO se construye en este branch (sus deps —
button.cpp, toolbar.cpp, etc. — son untracked en master). El subdirectorio
se anade pero protegido por FN_BUILD_GALLERY=OFF para no romper builds.

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

cpp/apps/primitives_gallery/demos.h

@@ -0,0 +1,37 @@
+#pragma once
+// Cada demo_xxx() renderiza una seccion completa para un primitivo.
+// Se llaman desde main.cpp en funcion del item seleccionado en el sidebar.
+
+namespace gallery {
+
+// --- Core ---
+void demo_button();
+void demo_icon_button();
+void demo_toolbar();
+void demo_modal();
+void demo_text_input();
+void demo_select();
+void demo_toast();
+void demo_tree_view();
+void demo_kpi_card();
+void demo_badge();
+void demo_empty_state();
+void demo_page_header();
+void demo_dashboard_panel();
+
+// --- Viz ---
+void demo_bar_chart();
+void demo_pie_chart();
+void demo_line_plot();
+void demo_scatter_plot();
+void demo_histogram();
+void demo_sparkline();
+void demo_graph();
+
+// --- Gfx ---
+void demo_shader_canvas();
+
+// --- Core (combined demo: text_editor + file_watcher) ---
+void demo_text_editor();
+
+} // namespace gallery