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.

scripts/run.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Run one scenario in the foreground and exit.
+#
+#   ./scripts/run.sh getvsload
+#   ./scripts/run.sh mergerefresh
+#   ./scripts/run.sh insert-identity
+#   ./scripts/run.sh insert-sequence
+#
+# Every scenario here is a CommandLineRunner against an in-memory H2 database with
+# spring.main.web-application-type=none, so there is no server to keep alive and nothing to
+# kill afterwards -- the process runs the scenario and exits on its own.
+set -eu
+
+PROFILE="${1:?usage: run.sh <getvsload|mergerefresh|insert-identity|insert-sequence>}"
+
+cd "$(dirname "$0")/.."
+mvn -q -B org.springframework.boot:spring-boot-maven-plugin:run \
+  -Dspring-boot.run.profiles="$PROFILE"