Thread-id table for engine frames

- frames live in a 256-slot table indexed by thread id, ThreadLocal only on slot collisions
- the single ThreadLocal probed long collision chains in the workers' thread-local maps

core/src/test/java/art/arcane/iris/generation/runtime/EngineThreadStateTest.java

@@ -49,6 +49,39 @@ public class EngineThreadStateTest {
         assertNull(state.current());
     }
 
+    @Test
+    public void threadsSharingATableSlotKeepSeparateFrames() throws InterruptedException {
+        EngineThreadState state = new EngineThreadState();
+        IrisEngine.GenerationRuntimeBinding mainBinding = new IrisEngine.GenerationRuntimeBinding(
+                mock(IrisEngine.class), mock(GenerationRuntime.class));
+        IrisEngine.GenerationRuntimeBinding otherBinding = new IrisEngine.GenerationRuntimeBinding(
+                mock(IrisEngine.class), mock(GenerationRuntime.class));
+        IrisEngine.GenerationRuntimeScope mainScope = state.open(mainBinding);
+        long slot = Thread.currentThread().threadId() & 255;
+        AtomicReference<Object> otherSeen = new AtomicReference<>();
+        AtomicReference<Object> otherAfterClose = new AtomicReference<>(otherBinding);
+        Thread colliding = null;
+        while (colliding == null) {
+            Thread candidate = new Thread(() -> {
+                IrisEngine.GenerationRuntimeScope scope = state.open(otherBinding);
+                otherSeen.set(state.binding());
+                scope.close();
+                otherAfterClose.set(state.current());
+            });
+            if ((candidate.threadId() & 255) == slot) {
+                colliding = candidate;
+            }
+        }
+        colliding.start();
+        colliding.join();
+
+        assertSame(otherBinding, otherSeen.get());
+        assertNull(otherAfterClose.get());
+        assertSame(mainBinding, state.binding());
+        mainScope.close();
+        assertNull(state.current());
+    }
+
     @Test
     public void framesAreThreadConfinedAndScopesRejectForeignClose() throws InterruptedException {
         EngineThreadState state = new EngineThreadState();