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/rpg/entity/entitytestfixture.c

@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "entitytestfixture.h"
+#include "rpg/overworld/map.h"
+#include "rpg/overworld/tile.h"
+#include "util/memory.h"
+
+void entityTestFixtureReset(void) {
+  memoryZero(ENTITIES, sizeof(ENTITIES));
+
+  memoryZero(&MAP, sizeof(map_t));
+  MAP.loaded = true;
+  MAP.chunkPosition = (chunkpos_t){ 0, 0, 0 };
+
+  chunk_t *chunk = &MAP.chunks[0];
+  chunk->position = (chunkpos_t){ 0, 0, 0 };
+  for(uint32_t i = 0; i < CHUNK_TILE_COUNT; i++) {
+    chunk->tiles[i] = (tile_t){ .shape = TILE_SHAPE_GROUND, .z = 0 };
+  }
+  for(uint8_t i = 0; i < CHUNK_ENTITY_COUNT_MAX; i++) {
+    chunk->entities[i] = 0xFF;
+  }
+
+  MAP.chunkOrder[0] = chunk;
+}