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/clean_output.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""Turn a raw mvn spring-boot:run capture into a clean, reproducible transcript.
+
+Strips JVM/build noise (sun.misc.Unsafe warnings, JAVA_TOOL_OPTIONS proxy banners) and the
+duplicate un-prefixed echo of each SQL statement that org.hibernate.SQL's show_sql=true prints
+to stdout in addition to the "Hibernate: ..." line the logger emits -- same text twice, so only
+the logger-prefixed copy is kept.
+"""
+import re
+import sys
+
+DROP_PREFIXES = (
+    "WARNING:",
+    "Picked up JAVA_TOOL_OPTIONS",
+)
+
+
+def clean(lines):
+    out = []
+    for line in lines:
+        stripped = line.rstrip("\n")
+        if any(stripped.startswith(p) for p in DROP_PREFIXES):
+            continue
+        # Drop the bare SQL echo line that show_sql=true prints without the "Hibernate: " prefix
+        # -- it is always immediately followed by the same text WITH the prefix.
+        out.append(stripped)
+    deduped = []
+    i = 0
+    while i < len(out):
+        cur = out[i]
+        nxt = out[i + 1] if i + 1 < len(out) else None
+        if nxt is not None and nxt == "Hibernate: " + cur:
+            i += 1  # skip the bare echo, keep the prefixed one on the next iteration
+            continue
+        deduped.append(out[i])
+        i += 1
+    return deduped
+
+
+if __name__ == "__main__":
+    src, dst = sys.argv[1], sys.argv[2]
+    with open(src, encoding="utf-8") as f:
+        lines = f.readlines()
+    cleaned = clean(lines)
+    with open(dst, "w", encoding="utf-8") as f:
+        f.write("\n".join(cleaned) + "\n")
+    print(f"{src}: {len(lines)} -> {dst}: {len(cleaned)} lines")