Add the transactions module, and move the migration project under migration-behavior/

The repository now aggregates two independent modules. migration-behavior/ is the
original project, moved unchanged; it stays on Spring Boot 4.0.6 / JDK 21 because
that is what the four published migration articles were verified against, and
upgrading it would silently invalidate output they quote. The article-tagged trees
are untouched, so links into a tag are unaffected.

transactions/  Companion code for "@Transactional in Spring: Propagation, Isolation,
and the Six Ways It Silently Does Nothing". Spring Boot 4.1.1 / JDK 25.

Every row of the propagation matrix is produced by calling the method and asking the
transaction manager what it did. The transaction NAME is the exhibit: a scope that
joined reports its caller's name, a scope that started its own reports its own.

Three things the transcripts settle:

  - Propagation.NESTED cannot be used with JpaTransactionManager. It fails twice,
    with two different messages, the second of which blames your JPA provider. The
    savepoint manager comes from the object the JpaDialect returns when it begins the
    transaction, and Hibernate's does not implement one. It works on
    DataSourceTransactionManager, because a savepoint is a JDBC concept -- shown
    working there rather than only failing here.
  - Catching a REQUIRED inner failure does not save the transaction. The inner scope
    has already marked it rollback-only, so the commit throws
    UnexpectedRollbackException from a place with no connection to the cause.
  - A checked exception commits, and so does a swallowed one. Those two do not merely
    fail to start a transaction; they commit work the code was abandoning.

19 contract tests, six captured transcripts, all regenerated by scripts/run-all.sh.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gip4srpzMwjgoba6uEfbr5

transactions/scripts/env.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+# Shared environment. Point JAVA_HOME at a JDK 25 (or newer) installation.
+: "${JAVA_HOME:?set JAVA_HOME to a JDK 25+ installation}"
+export PATH="$JAVA_HOME/bin:$PATH"
+MVN="${MVN:-mvn}"
+JAR="target/transactions-1.0.0.jar"
+APP_PORT="${APP_PORT:-8081}"
+LOG="${LOG:-/tmp/transactions-demo.log}"
+
+# Strip machine-specific noise from committed transcripts.
+clean() { grep -v "Picked up JAVA_TOOL_OPTIONS" | grep -v "^OpenJDK 64-Bit Server VM warning"; }
+
+# Reduce a Spring log line to its message, so transcripts diff cleanly between runs.
+tidy() { sed -E 's/^[0-9T:.-]+Z +//; s/^[A-Z]+ +[0-9]+ --- \[[^]]*\] \[[^]]*\] +//; s/ +: /: /'; }
+
+# Start detached and block until it answers. Deliberately NOT setsid: setsid forks when it is
+# not already a process-group leader, so $! would name a process that exits immediately and
+# the JVM would survive every later stop_app -- holding the port, so the next scenario fails
+# to bind and curl answers from the previous one. That reads exactly like the configuration
+# under test having had no effect.
+start_app() {
+  stop_app
+  mkdir -p target
+  nohup java -jar "$JAR" "$@" > "$LOG" 2>&1 < /dev/null &
+  echo $! > target/app.pid
+  for _ in $(seq 1 60); do
+    curl -s -o /dev/null "http://127.0.0.1:${APP_PORT}/tx/silent" 2>/dev/null && return 0
+    kill -0 "$(cat target/app.pid)" 2>/dev/null || { echo "JVM exited during startup:"
+      tail -20 "$LOG"; return 1; }
+    sleep 1
+  done
+  echo "application did not answer"; tail -20 "$LOG"; return 1
+}
+
+# Stop by recorded PID. Never by pattern: `ps | grep transactions` also matches the shell
+# running this script, because that string is on its own command line.
+stop_app() {
+  if [ -f target/app.pid ]; then
+    pid=$(cat target/app.pid)
+    if [ -n "$pid" ] && grep -qa "transactions" "/proc/$pid/cmdline" 2>/dev/null; then
+      kill -9 "$pid" 2>/dev/null || true
+      wait "$pid" 2>/dev/null || true
+    fi
+    rm -f target/app.pid
+  fi
+  for _ in $(seq 1 40); do
+    if ! (exec 3<>/dev/tcp/127.0.0.1/"${APP_PORT}") 2>/dev/null; then break; fi
+    sleep 0.25
+  done
+  exec 3<&- 2>/dev/null || true
+}