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

src/dusk/scene/mainmenu/scenemainmenu.c

@@ -0,0 +1,24 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "scenemainmenu.h"
+
+errorret_t sceneMainMenuInit(scenedata_t *sceneData) {
+  errorOk();
+}
+
+errorret_t sceneMainMenuUpdate(scenedata_t *sceneData) {
+  errorOk();
+}
+
+errorret_t sceneMainMenuRender(scenedata_t *sceneData) {
+  errorOk();
+}
+
+errorret_t sceneMainMenuDispose(scenedata_t *sceneData) {
+  errorOk();
+}