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-all.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Regenerate every file in docs/output/ from a real run. This is the script referenced by
+# "regenerated by one command" in the post and the README -- if it stops producing the same
+# shape of output, the docs are wrong until it's fixed, not the other way around.
+set -eu
+
+cd "$(dirname "$0")/.."
+RAW_DIR="$(mktemp -d)"
+trap 'rm -rf "$RAW_DIR"' EXIT
+
+run_one() {
+  local profile="$1" outfile="$2"
+  echo "=== running profile: $profile ===" >&2
+  mvn -q -B org.springframework.boot:spring-boot-maven-plugin:run \
+    -Dspring-boot.run.profiles="$profile" 2>&1 \
+    | grep -v '^Picked up JAVA_TOOL_OPTIONS' \
+    | grep -v '^WARNING:' \
+    > "$RAW_DIR/$profile.raw.txt"
+  python3 scripts/clean_output.py "$RAW_DIR/$profile.raw.txt" "docs/output/$outfile"
+}
+
+run_one getvsload        get-vs-load.txt
+run_one mergerefresh      merge-vs-refresh.txt
+run_one insert-identity   insert-identity.txt
+run_one insert-sequence   insert-sequence.txt
+
+echo "docs/output/ regenerated." >&2