Loader spawn position check

- The modded chunk generator hands VolmLib the bound loader for chunk-generation creature checks
- Each loader build tests its spawn position check: Forge and NeoForge fire MobSpawnEvent.PositionCheck and honor a denial, Fabric applies the vanilla rule and obstruction checks

adapters/fabric/src/test/java/art/arcane/volmlib/nativelib/minecraft26_2/fabric/NativeFabricSpawnPositionTest.java

@@ -0,0 +1,43 @@
+package art.arcane.volmlib.nativelib.minecraft26_2.fabric;
+
+import art.arcane.volmlib.nativelib.modded.NativeLoaderOptions;
+import net.minecraft.SharedConstants;
+import net.minecraft.server.Bootstrap;
+import net.minecraft.world.entity.EntitySpawnReason;
+import net.minecraft.world.entity.Mob;
+import net.minecraft.world.level.ServerLevelAccessor;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import java.util.function.BooleanSupplier;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class NativeFabricSpawnPositionTest {
+    @BeforeClass
+    public static void bootstrap() {
+        SharedConstants.tryDetectVersion();
+        Bootstrap.bootStrap();
+    }
+
+    @Test
+    public void chunkGenerationSpawnsUseTheVanillaRulesAndObstructionChecks() {
+        NativeFabricLoader loader = new NativeFabricLoader(
+                new NativeLoaderOptions("irisworldgen", "treefeller", BooleanSupplier::getAsBoolean));
+        ServerLevelAccessor level = mock(ServerLevelAccessor.class);
+        Mob mob = mock(Mob.class);
+        when(mob.checkSpawnRules(level, EntitySpawnReason.CHUNK_GENERATION)).thenReturn(true);
+        when(mob.checkSpawnObstruction(level)).thenReturn(true);
+        assertTrue(loader.checkSpawnPosition(mob, level, EntitySpawnReason.CHUNK_GENERATION));
+
+        when(mob.checkSpawnObstruction(level)).thenReturn(false);
+        assertFalse(loader.checkSpawnPosition(mob, level, EntitySpawnReason.CHUNK_GENERATION));
+
+        when(mob.checkSpawnObstruction(level)).thenReturn(true);
+        when(mob.checkSpawnRules(level, EntitySpawnReason.CHUNK_GENERATION)).thenReturn(false);
+        assertFalse(loader.checkSpawnPosition(mob, level, EntitySpawnReason.CHUNK_GENERATION));
+    }
+}