Take chunk uses through pins

- Every getChunk(..).use() site calls Mantle.useChunk, which cannot fail because a save closed the chunk in between
- Covered-chunk cleanup, custom block and static object layers take their chunk through useChunk
- Discovered flag writes go through Mantle.flag instead of an unpinned chunk
- Test mantles stub useChunk the way the real one behaves

adapters/modded-common/src/test/java/art/arcane/iris/modded/ModdedSavedBiomeConsumersTest.java

@@ -70,8 +70,7 @@ public class ModdedSavedBiomeConsumersTest {
         when(engine.getWorld()).thenReturn(mock(IrisWorld.class));
         when(engineMantle.getMantle()).thenReturn(mantle);
         when(mantle.isChunkLoaded(2, -1)).thenReturn(true);
-        when(mantle.getChunk(2, -1)).thenReturn(chunk);
-        when(chunk.use()).thenReturn(chunk);
+        when(mantle.useChunk(2, -1)).thenReturn(chunk);
         BiomeEnvironment environment = environment();
         when(engine.getSurfaceBiomeEnvironment(40, -8))
                 .thenThrow(new SavedBiomeUnavailableException("Loading", true))