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/CMakeLists.txt

@@ -0,0 +1,60 @@
+add_imgui_app(primitives_gallery
+    main.cpp
+    demo.cpp
+    demos_core.cpp
+    demos_viz.cpp
+    demos_graph.cpp
+    demos_gfx.cpp
+    demos_text_editor.cpp
+    # text_editor + file_watcher (issue 0025)
+    ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp
+    ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit/TextEditor.cpp
+    # Core primitives demoed (tokens vive en fn_framework)
+    ${CMAKE_SOURCE_DIR}/functions/core/fullscreen_window.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/page_header.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/dashboard_panel.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/badge.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/empty_state.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/button.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/icon_button.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/toolbar.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/modal_dialog.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/text_input.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/select.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/toast.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/tree_view.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/process_runner.cpp
+    # Viz primitives demoed
+    ${CMAKE_SOURCE_DIR}/functions/viz/kpi_card.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/bar_chart.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/pie_chart.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/line_plot.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/scatter_plot.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/histogram.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/sparkline.cpp
+    # Graph stack (instanced GPU + Barnes-Hut + spatial hash)
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_types.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_renderer.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_force_layout.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/graph_viewport.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/graph_spatial_hash.cpp
+    # GL loader (Linux no-op, Windows wglGetProcAddress)
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_loader.cpp
+    # Shader stack (shader_canvas demo)
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_shader.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/gl_framebuffer.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/fullscreen_quad.cpp
+    ${CMAKE_SOURCE_DIR}/functions/gfx/shader_canvas.cpp
+)
+target_include_directories(primitives_gallery PRIVATE
+    ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit
+)
+
+if(WIN32)
+    target_link_libraries(primitives_gallery PRIVATE opengl32)
+endif()
+
+if(WIN32)
+    set_target_properties(primitives_gallery PROPERTIES WIN32_EXECUTABLE TRUE)
+endif()