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/main.cpp

@@ -49,6 +49,9 @@ static const DemoEntry k_demos[] = {
     {"text_editor",     "text_editor",     "Core", &gallery::demo_text_editor},     // wave 1
     {"file_watcher",    "file_watcher",    "Core", &gallery::demo_file_watcher},    // wave 1
     {"process_runner",  "process_runner",  "Core", &gallery::demo_process_runner},
+    {"tween",           "tween_curves",    "Core", &gallery::demo_tween},
+    {"bezier_editor",   "bezier_editor",   "Core", &gallery::demo_bezier_editor},
+    {"timeline",        "timeline",        "Core", &gallery::demo_timeline},
     // Viz
     {"bar_chart",       "bar_chart",       "Viz",  &gallery::demo_bar_chart},
     {"pie_chart",       "pie_chart",       "Viz",  &gallery::demo_pie_chart},