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/docs/05-six-silent-failures.md

@@ -0,0 +1,95 @@
+[← NESTED and JPA](04-nested-and-jpa.md) · [Index](../README.md) · [Isolation →](06-isolation-and-readonly.md)
+
+# 5. Six ways `@Transactional` silently does nothing
+
+Sources: [`SilentlyNonTransactional`](../src/main/java/com/ankurm/tx/service/SilentlyNonTransactional.java),
+[`NotABean`](../src/main/java/com/ankurm/tx/service/NotABean.java).
+Transcript: [`04-silent-failures.txt`](output/04-silent-failures.txt).
+
+Row 0 is the control: the same annotated method reached through the proxy reports
+`actualTransactionActive=true`. The mechanism works. These six do not use it.
+
+## 1. Self-invocation
+
+```java
+public String entryPoint() {
+    return annotatedButCalledInternally();   // this.  -> no proxy -> no transaction
+}
+
+@Transactional
+public String annotatedButCalledInternally() { ... }
+```
+
+`actualTransactionActive=false`. The proxy wraps the *object*, not its methods; a call the
+object makes to itself never leaves it.
+
+**Fix:** move the method to another bean. Self-injection and `AopContext.currentProxy()` both
+work and are both worse.
+
+## 2. A private method
+
+`@Transactional` on a private method is legal Java and inert: a CGLIB proxy advises by
+overriding, and private methods cannot be overridden. IntelliJ warns; the compiler does not.
+
+## 3. A checked exception commits
+
+```java
+@Transactional
+public void checkedExceptionCommits(String id) throws Exception {
+    accounts.save(new Account(id, 999));
+    throw new Exception("checked -- this does NOT trigger rollback");
+}
+```
+
+```
+"3-checked-exception": { "threw": "Exception", "rowSurvived": true,
+                         "verdict": "COMMITTED despite the exception" }
+```
+
+The default rollback rule is `RuntimeException` or `Error`. A checked exception propagates to
+the caller **and the transaction commits on the way out**.
+
+**Fix:** `@Transactional(rollbackFor = Exception.class)`.
+
+## 4. Swallowing the exception
+
+```java
+@Transactional
+public void swallowsException(String id) {
+    accounts.save(new Account(id, 555));
+    try { throw new IllegalStateException("something went wrong"); }
+    catch (RuntimeException ex) { /* handled */ }
+}
+```
+
+Nothing propagates, so the interceptor sees a normal return and commits. The write survives the
+failure the code appeared to handle.
+
+Failures 3 and 4 are the dangerous pair: they do not merely fail to start a transaction, they
+**commit work the code was trying to abandon**.
+
+## 5. Called from `@PostConstruct`
+
+```
+"5-called-from-post-construct": { "transactionActiveDuringPostConstruct": false }
+```
+
+The proxy does not exist while the bean is still initialising, so there is nothing to intercept
+the call. The reference documentation says not to rely on it; this measures what actually
+happens.
+
+**Fix:** `ApplicationReadyEvent` or `InitializingBean` on a *different* bean.
+
+## 6. An object created with `new`
+
+No container, no proxy, no transaction. Survives code review easily because the annotation is
+right there on the method.
+
+## The one-line check
+
+```java
+TransactionSynchronizationManager.isActualTransactionActive()
+```
+
+Drop it into the method you believe is transactional. If it prints `false`, you have one of
+these six and no amount of reasoning about propagation will help.