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.
