Refactor simple pathing timeline logic into dedicated class with unit tests

Fix logic for user-set interpolators

src/main/java/com/replaymod/simplepathing/properties/ExplicitInterpolationProperty.java

@@ -0,0 +1,44 @@
+package com.replaymod.simplepathing.properties;
+
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import com.replaymod.replaystudio.pathing.property.AbstractProperty;
+import com.replaymod.replaystudio.pathing.property.PropertyPart;
+import lombok.NonNull;
+import org.apache.commons.lang3.ObjectUtils;
+
+import java.io.IOException;
+import java.util.Collection;
+import java.util.Collections;
+
+/**
+ * Property indicating that the next path segment's interpolator is fixed
+ */
+public class ExplicitInterpolationProperty extends AbstractProperty<Object> {
+    public static final ExplicitInterpolationProperty PROPERTY = new ExplicitInterpolationProperty();
+
+    private ExplicitInterpolationProperty() {
+        super("interpolationFixed", "<internal>", null, new Object());
+    }
+
+    @Override
+    public Collection<PropertyPart<Object>> getParts() {
+        return Collections.emptyList();
+    }
+
+    @Override
+    public void applyToGame(Object value, @NonNull Object replayHandler) {
+        // dummy property, do nothing
+    }
+
+    @Override
+    public void toJson(JsonWriter writer, Object value) throws IOException {
+        writer.nullValue();
+    }
+
+    @Override
+    public Object fromJson(JsonReader reader) throws IOException {
+        reader.nextNull();
+        return ObjectUtils.NULL;
+    }
+}