Clamp keyframe interpolation to last value, add main menu scene/UI, battle HUD, and expanded test coverage

- keyframeGetValue now returns the last keyframe's value for times at or
  beyond it, fixes a missing util/math.h include, and asserts keyframes are
  sorted by time; adds test/animation/test_keyframe.c
- Adds mainmenu scene/UI and a battle HUD UI frame
- Adds save autosave-related fields and battle scene tweaks
- Adds headless test coverage for cutscenes, entities, and map areas

test/CMakeLists.txt

@@ -3,6 +3,7 @@
 # This software is released under the MIT License.
 # https://opensource.org/licenses/MIT
 
+add_subdirectory(animation)
 add_subdirectory(assert)
 add_subdirectory(asset)
 add_subdirectory(error)