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

probe/src/test/java/art/arcane/iris/probe/ObjectContinuationProbeTest.java

@@ -290,7 +290,7 @@ public final class ObjectContinuationProbeTest {
         }
         GenerationHistoryRuntimeRouter router = engine.getGenerationHistoryRuntimeRouter().orElseThrow();
         try (GenerationHistoryRuntimeRouter.SavedChunkMantle saved = router.openSavedChunkMantle(0, 0)) {
-            MantleChunk<Matter> chunk = saved.mantle().getChunk(0, 0).use();
+            MantleChunk<Matter> chunk = saved.mantle().useChunk(0, 0);
             try {
                 for (Map.Entry<Position, String> edit : edits.entrySet()) {
                     Position position = edit.getKey();