Fix combine bound axis not being part of primary key

This did not impact results, as it only resulted in looser bounds and some bounds not being stored.

src/db.rs

@@ -226,7 +226,7 @@ impl SqliteLayoutDB {
     pub fn add_merge_lower_bounds(&mut self, lower_bounds: Vec<MergeLowerBound>) -> Result<()> {
         let transaction = self.connection.transaction()?;
         for lower_bound in &lower_bounds {
-            transaction.execute("INSERT INTO merge_lower_bounds (left_layout_id, right_layout_id, axis, price) VALUES (?1, ?2, ?3, ?4) ON CONFLICT(left_layout_id, right_layout_id) DO UPDATE SET price = ?4",
+            transaction.execute("INSERT INTO merge_lower_bounds (left_layout_id, right_layout_id, axis, price) VALUES (?1, ?2, ?3, ?4) ON CONFLICT(left_layout_id, right_layout_id, axis) DO UPDATE SET price = ?4",
                                 params![lower_bound.left_layout_id as u32,
                                 lower_bound.right_layout_id as u32,
                                 if lower_bound.y_axis {1u32} else {0u32},