feat(primitives_gallery): demos for tween_curves + bezier_editor + timeline

Adds three new demos to the Core section of primitives_gallery:

- demo_tween: dropdown of all 16 Ease modes + animated plot showing the
  curve and a moving marker that traverses t=0..1 in a loop.
- demo_bezier_editor: live editor with reset + ease-out / ease-in-out
  presets, current control points displayed, slider over t showing
  bezier_eval(curve, t).
- demo_timeline: 2 tracks (hue, amp) with mixed eases, live progress bars
  showing track_value_at(current_time) updating each frame.

Wires the three demos into k_demos[] in main.cpp and adds the new sources
(plus the three function .cpp files) to CMakeLists.txt.

cpp/apps/primitives_gallery/CMakeLists.txt

@@ -8,6 +8,11 @@ add_imgui_app(primitives_gallery
     demos_text_editor.cpp
     demos_gl_texture.cpp
     demos_extras.cpp
+    # animation primitives (issue 0031)
+    demos_animation.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/tween_curves.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/bezier_editor.cpp
+    ${CMAKE_SOURCE_DIR}/functions/core/timeline.cpp
     # text_editor + file_watcher (issue 0025)
     ${CMAKE_SOURCE_DIR}/functions/core/text_editor.cpp
     ${CMAKE_SOURCE_DIR}/functions/core/file_watcher.cpp