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/merge-vs-refresh.txt

@@ -0,0 +1,41 @@
+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) <- [Robert C. Martin]
+binding parameter (2:VARCHAR) <- [Clean Code]
+binding parameter (3:BIGINT) <- [0]
+binding parameter (4:BIGINT) <- [1]
+SEED: inserted Book{id=1, title=Clean Code, author=Robert C. Martin, version=0}
+--- Step 1: load the row, then close the session (entity is now detached) ---
+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]
+detached instance in hand: Book{id=1, title=Clean Code, author=Robert C. Martin, version=0}
+--- Step 2: a second, independent session edits the same row and commits ---
+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]
+Hibernate: /* update for com.ankurm.hibernatedemo.model.Book */update book set author=?,title=?,version=? where id=? and version=?
+binding parameter (1:VARCHAR) <- [Robert C. Martin]
+binding parameter (2:VARCHAR) <- [Clean Code (2nd Edition)]
+binding parameter (3:BIGINT) <- [1]
+binding parameter (4:BIGINT) <- [1]
+binding parameter (5:BIGINT) <- [0]
+second session committed: Book{id=1, title=Clean Code (2nd Edition), author=Robert C. Martin, version=1} -- version column has now advanced in the database
+--- Step 3: mutate the ORIGINAL detached instance (still holding the OLD version) and merge() it ---
+detached instance before merge (note the version and title are both stale): Book{id=1, title=Clean Code, author=Robert C. Martin (Uncle Bob), version=0}
+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]
+merge() threw jakarta.persistence.OptimisticLockException: Row was already updated or deleted by another transaction for entity [com.ankurm.hibernatedemo.model.Book with id '1']
+the title change from Step 2 survives untouched -- merge() refused to apply a write built on a stale version
+--- Step 4: refresh() on a MANAGED entity with an unflushed local edit ---
+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]
+before refresh(): Book{id=1, title=Clean Code (2nd Edition), author=SOMEONE ELSE ENTIRELY (never flushed), version=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]
+after refresh():  Book{id=1, title=Clean Code (2nd Edition), author=Robert C. Martin, version=1} -- the local edit is gone, no exception was thrown