feat(0132): add terminal_panel demo (oneshot, interactive, readonly)

Add demos_terminal.cpp con 3 demos del modulo terminal_panel:
- Demo 1: one-shot spawn bash, echo hello; date; ls /tmp
- Demo 2: terminal interactivo bash/cmd.exe con input box
- Demo 3: readonly tail -f con boton append

Actualiza CMakeLists.txt con ansi_parser + terminal_panel + -lutil (Linux).
Actualiza demos.h + main.cpp con entry "Terminal" en el sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CMakeLists.txt

@@ -91,6 +91,12 @@ add_imgui_app(primitives_gallery
     ${CMAKE_SOURCE_DIR}/functions/gfx/mesh_gpu.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/orbit_camera.cpp
     ${CMAKE_SOURCE_DIR}/functions/viz/mesh_viewer.cpp
+    # terminal_panel module (issue 0132)
+    demos_terminal.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/ansi_parser.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/terminal_panel/terminal_panel.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/terminal_panel/terminal_panel_linux.cpp
+    ${CMAKE_SOURCE_DIR}/functions/viz/terminal_panel/terminal_panel_windows.cpp
 )
 target_include_directories(primitives_gallery PRIVATE
     ${CMAKE_SOURCE_DIR}/vendor/imgui_text_edit
@@ -108,3 +114,8 @@ endif()
 if(WIN32)
     set_target_properties(primitives_gallery PROPERTIES WIN32_EXECUTABLE TRUE)
 endif()
+
+# terminal_panel: forkpty requiere -lutil en Linux.
+if(NOT WIN32)
+    target_link_libraries(primitives_gallery PRIVATE util)
+endif()