Add hibernate-demo: get() vs load(), merge() vs refresh(), inserting objects (Hibernate 7.4.1.Final + Spring Boot 4.1.0)

Adds a JUnit test suite (GetVsGetReferenceTest, MergeRefreshTest, OptimisticLockTest,
IdentityBatchTest, SequenceBatchTest, AllocationSizeSweepTest, BatchSizeSweepTest) so every
surprising behavior described in the three companion posts has a reproducible test, alongside
the original CommandLineRunner scenarios. Rewrites all three doc chapters and the README around
the new experiments: the get()/getReference() same-session matrix, the merge()/refresh()
experiments (including exactly when OptimisticLockException surfaces and a corrected LAZY-plus-
cascade merge() result), and two new sweeps (allocationSize, batch_size) for batch inserts.

src/test/java/com/ankurm/hibernatedemo/SequenceBatchTest.java

@@ -0,0 +1,51 @@
+package com.ankurm.hibernatedemo;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import com.ankurm.hibernatedemo.model.WidgetSequence;
+import jakarta.persistence.EntityManager;
+import jakarta.persistence.EntityManagerFactory;
+import org.hibernate.SessionFactory;
+import org.hibernate.stat.Statistics;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+
+/**
+ * Backs ankurm.com post 4861. Docs: docs/03-inserting-objects.md.
+ * Run with {@code ./mvnw -Dtest=SequenceBatchTest test}.
+ *
+ * <p>{@link WidgetSequence} sets {@code allocationSize = 25}, matching
+ * {@code hibernate.jdbc.batch_size} in application.yml. See {@code AllocationSizeSweepTest} for
+ * what happens when the two are deliberately mismatched.
+ */
+@SpringBootTest
+class SequenceBatchTest {
+
+    private static final int ROW_COUNT = 30;
+
+    @Autowired
+    private EntityManagerFactory emf;
+
+    @Test
+    void sequenceGeneratorAllowsBatching_fourPreparedStatementsForThirtyRows() {
+        SessionFactory sessionFactory = emf.unwrap(SessionFactory.class);
+        Statistics stats = sessionFactory.getStatistics();
+        stats.clear();
+
+        EntityManager em = emf.createEntityManager();
+        em.getTransaction().begin();
+        for (int i = 1; i <= ROW_COUNT; i++) {
+            em.persist(new WidgetSequence("sequence-" + i));
+        }
+        em.getTransaction().commit();
+        em.close();
+
+        assertThat(stats.getEntityInsertCount()).isEqualTo(ROW_COUNT);
+        assertThat(stats.getPrepareStatementCount())
+                .as("30 rows at batch_size=25 is 2 insert batches (25 + 5); allocationSize=25 "
+                        + "means the first 25 ids come from one sequence call and the remaining "
+                        + "5 force a second -- 2 insert batches + 2 sequence calls = 4")
+                .isEqualTo(4);
+    }
+}