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/demos.h

@@ -21,6 +21,9 @@ void demo_dashboard_panel();
 void demo_text_editor();      // wave 1, issue 0025
 void demo_file_watcher();     // wave 1, issue 0025
 void demo_process_runner();
+void demo_tween();            // issue 0031
+void demo_bezier_editor();    // issue 0031
+void demo_timeline();         // issue 0031
 
 // --- Viz ---
 void demo_bar_chart();