test: characterize TEdgePart

Freezes the single trait default before the port: conductsRedstone returns
false, TEdgePart extends TSlottedPart, and the $class static holds the
default rather than dispatching, so an overriding part gets its own value
from a direct call but false from the bridge.

The member is abstract on the reference interface, so a Java class cannot
inherit it today; the test part mirrors the forwarder Scala emits instead,
which keeps the file compiling before and after the port.

Adds a frozen Scala 2.11.5 consumer compiled against the reference dev jar
at f2f1870. OpenComputers is the only consumer, and it links against
$init$ rather than conductsRedstone.

61 plain-JVM tests, all passing against the untouched Scala.

src/test/java/codechicken/multipart/EdgePartCharacterizationTest.java

@@ -0,0 +1,58 @@
+package codechicken.multipart;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+class EdgePartCharacterizationTest {
+
+    @Test
+    void edgePartsDoNotConductRedstoneByDefault() {
+        assertFalse(new ForwardingPart().conductsRedstone());
+    }
+
+    /** The bridge holds the trait default rather than dispatching, so an override does not change what it returns. */
+    @Test
+    void deprecatedBridgeReturnsTheTraitDefaultEvenWhenOverridden() {
+        assertFalse(TEdgePart$class.conductsRedstone(new ForwardingPart()));
+        assertTrue(new ConductingPart().conductsRedstone());
+        assertFalse(TEdgePart$class.conductsRedstone(new ConductingPart()));
+    }
+
+    @Test
+    void edgePartIsASlottedPart() {
+        ForwardingPart part = new ForwardingPart();
+
+        assertTrue(part instanceof TSlottedPart);
+        assertEquals(1 << 15, part.getSlotMask());
+    }
+
+    /** Mirrors the forwarder Scala emits for a class mixing in the trait without overriding it. */
+    private static class ForwardingPart extends TMultiPart implements TEdgePart {
+
+        @Override
+        public String getType() {
+            return "test:edge";
+        }
+
+        @Override
+        public int getSlotMask() {
+            return 1 << 15;
+        }
+
+        @Override
+        public boolean conductsRedstone() {
+            return TEdgePart$class.conductsRedstone(this);
+        }
+    }
+
+    private static final class ConductingPart extends ForwardingPart {
+
+        @Override
+        public boolean conductsRedstone() {
+            return true;
+        }
+    }
+}