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.

docs/output/get-vs-load.txt

@@ -0,0 +1,46 @@
+Hibernate: create global temporary table HTE_book(rn_ integer not null, id bigint, version bigint, author varchar(255), title varchar(255), primary key (rn_)) transactional
+Hibernate: create global temporary table HTE_widget_sequence(rn_ integer not null, id bigint, name varchar(255), primary key (rn_)) transactional
+Hibernate: create table book (id bigint not null, author varchar(255), title varchar(255), version bigint not null, primary key (id))
+Hibernate: create table widget_identity (id bigint generated by default as identity, name varchar(255), primary key (id))
+Hibernate: create table widget_sequence (id bigint not null, name varchar(255), primary key (id))
+Hibernate: create sequence book_seq start with 1 increment by 50
+Hibernate: create sequence widget_seq start with 1 increment by 25
+Hibernate: select next value for book_seq
+Hibernate: /* insert for com.ankurm.hibernatedemo.model.Book */insert into book (author,title,version,id) values (?,?,?,?)
+binding parameter (1:VARCHAR) <- [Joshua Bloch]
+binding parameter (2:VARCHAR) <- [Effective Java]
+binding parameter (3:BIGINT) <- [0]
+binding parameter (4:BIGINT) <- [1]
+SEED: inserted Book id=1
+--- Step 1: session.get() on an existing id ---
+about to call session.get(Book.class, 1)
+Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
+binding parameter (1:BIGINT) <- [1]
+get() returned: Book{id=1, title=Effective Java, author=Joshua Bloch, version=0}
+--- Step 2: session.get() on a missing id ---
+about to call session.get(Book.class, 999001)
+Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
+binding parameter (1:BIGINT) <- [999001]
+get() returned: null (no exception thrown)
+--- Step 3: session.getReference() on an existing id ---
+getReference() returned proxy of class com.ankurm.hibernatedemo.model.Book$HibernateProxy -- no SELECT above this line
+now calling proxy.getTitle() ...
+Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
+binding parameter (1:BIGINT) <- [1]
+getTitle() returned 'Effective Java' -- the SELECT for this ran just above this line
+--- Step 4: session.getReference() on a missing id ---
+getReference() returned a proxy for a row that does not exist -- no exception yet: com.ankurm.hibernatedemo.model.Book$HibernateProxy
+Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
+binding parameter (1:BIGINT) <- [999001]
+accessing the proxy threw jakarta.persistence.EntityNotFoundException: No row with the given identifier exists for entity [com.ankurm.hibernatedemo.model.Book with id '999001']
+--- Step 5: proxy accessed after its session is closed ---
+session closed. proxy in hand: com.ankurm.hibernatedemo.model.Book$HibernateProxy
+accessing the proxy after close threw org.hibernate.LazyInitializationException: Could not initialize proxy [com.ankurm.hibernatedemo.model.Book#1] - no session
+--- Step 6: proxy identity vs a real loaded instance ---
+Hibernate: select b1_0.id,b1_0.author,b1_0.title,b1_0.version from book b1_0 where b1_0.id=?
+binding parameter (1:BIGINT) <- [1]
+real.getClass()  = com.ankurm.hibernatedemo.model.Book
+proxy.getClass() = com.ankurm.hibernatedemo.model.Book$HibernateProxy
+proxy instanceof Book.class:            true
+real.getClass() == proxy.getClass():    false
+real.equals(proxy) before proxy access: false