chore: import upstream snapshot with attribution

tests/fixtures/sample_alter_fk.sql

@@ -0,0 +1,12 @@
+CREATE TABLE customers (
+  id SERIAL PRIMARY KEY,
+  name TEXT NOT NULL
+);
+
+CREATE TABLE orders (
+  id SERIAL PRIMARY KEY,
+  customer_id INT,
+  total NUMERIC
+);
+
+ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(id);