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/06-isolation-and-readonly.md

@@ -0,0 +1,62 @@
+[← Six silent failures](05-six-silent-failures.md) · [Index](../README.md)
+
+# 6. Isolation, read-only, and settings that are ignored
+
+Transcript: [`05-isolation.txt`](output/05-isolation.txt).
+
+## The levels
+
+| Level | Prevents | Cost |
+|---|---|---|
+| `READ_UNCOMMITTED` | nothing | lowest |
+| `READ_COMMITTED` | dirty reads | low — the default on Postgres, SQL Server, Oracle |
+| `REPEATABLE_READ` | dirty + non-repeatable reads | medium — the default on MySQL |
+| `SERIALIZABLE` | all of the above + phantoms | highest |
+
+`ISOLATION_DEFAULT` means "whatever the connection already has", which is the database's
+default and not something Spring chooses.
+
+## Settings apply only where the transaction starts
+
+The finding worth taking away:
+
+```
+  @Transactional(isolation = SERIALIZABLE) [outer]   isolation=SERIALIZABLE
+    REQUIRED inner joining it                        isolation=SERIALIZABLE
+
+  plain @Transactional [outer]                       isolation=default
+    inner declaring READ_UNCOMMITTED                 isolation=default
+```
+
+The inner method declares `@Transactional(isolation = READ_UNCOMMITTED)` and gets the default.
+It joined an existing physical transaction, whose isolation was fixed when it began. The
+declaration is not rejected, nothing is logged, and it simply has no effect.
+
+The same is true of `readOnly` and `timeout` on a participating scope. All three are properties
+of a physical transaction, and a participating scope does not have one of its own.
+
+Confirmed in the transaction manager's own log: `ISOLATION_SERIALIZABLE` appears on
+`Creating new transaction` lines and never on `Participating in existing transaction` lines,
+because participation carries no settings.
+
+**Make it loud:** set `validateExistingTransaction=true` on the transaction manager and a
+mismatched declaration becomes an exception instead of a silent no-op. It is off by default.
+
+## `readOnly` does less than its name suggests
+
+`readOnly = true` is a **hint**. It does not make the database reject writes. What it does:
+
+- Sets the JDBC connection read-only flag, which some drivers act on and others ignore.
+- Puts Hibernate's `FlushMode` to `MANUAL`, so the persistence context does not dirty-check or
+  flush — which is where the real performance benefit comes from on read paths.
+- Lets a routing datasource send the transaction to a replica.
+
+A write inside a `readOnly` transaction may silently do nothing (never flushed) or may fail,
+depending on the driver. "Silently does nothing" is the more common and more dangerous outcome,
+and it is a genuine seventh entry for [chapter 5](05-six-silent-failures.md)'s list.
+
+## `timeout`
+
+Seconds, enforced by Spring for the operations it controls and passed to the JDBC statement
+timeout where supported. Applies only to a scope that starts a transaction — same rule as
+above.