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.h

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "rpg/entity/entity.h"
+
+/**
+ * Resets ENTITIES[] to all-empty and builds a single minimal, synchronously
+ * "loaded" chunk at chunk position (0, 0, 0) on the MAP global, entirely
+ * TILE_SHAPE_GROUND at local Z 0. Sufficient for entityWalk/Turn/Run, chunk
+ * bookkeeping, and NPC movement tests -- does not exercise the real async
+ * chunk-loading pipeline (mapChunkLoad).
+ *
+ * World positions with x/y in [0, CHUNK_WIDTH-1]/[0, CHUNK_HEIGHT-1] and
+ * z == 0 fall within the fixture chunk. Individual tests may override
+ * specific columns afterward (e.g. to place a ramp).
+ */
+void entityTestFixtureReset(void);