[magnusp]

Fixes #210

Names the savepoint state transitions in LibsqlConnection. The three fields
(autoCommit, transactionStartedBySavepoint, savepointNames) still encode
one state machine, but each transition now has a name and one caller.

What changed

• beginSavepointTransaction — the savepoint sent in auto-commit opens the
  stream and owns the transaction; no BEGIN is sent, and a failed savepoint
  closes the fresh stream and leaves auto-commit on.
• discardSavepointsAfter — a rollback cancels the savepoints created after
  the target; the target stays active.
• closeTransactionOwnedBySavepoint — a release drops the released savepoint
  and its nested ones, and closes the stream only when that savepoint is the
  outermost one that started the transaction.
• requireManualCommit(String action) — builds Cannot <action> when in autoCommit mode, so commit, rollback, rollback-to-savepoint, and
  release-savepoint each keep their distinct wording.
• savepointSqlName moved to LibsqlSavepoint.sqlNameOf. Same messages, and
  the savepoint type now owns savepoint naming while the connection keeps the
  transitions.
• beginTransaction folded into its only caller ensureTransactionStarted,
  and the redundant second lock acquisition around the setSavepoint(String)
  name guard dropped. Both are behavior-preserving.

Preserved

• A savepoint in auto-commit starts a transaction without BEGIN; releasing
  the outermost savepoint commits and closes the stream.
• A failed implicit savepoint leaves auto-commit on and closes the fresh stream.
• Rollback and release in auto-commit send nothing and name the mode.
• setSavepoint(null) throws; setSavepoint() does not.
• Duplicate names resolve by lastIndexOf; the list is kept.
• Request order, messages, and lock discipline (stateLock held by every
  transition) unchanged. No test file and no pom.xml change.

Diff

 libsql-jdbc/src/main/java/com/github/magnusp/libsql/jdbc/LibsqlConnection.java | 166 ++++++++++-----------
 libsql-jdbc/src/main/java/com/github/magnusp/libsql/jdbc/LibsqlSavepoint.java  |  27 ++++
 2 files changed, 103 insertions(+), 90 deletions(-)

Line counts: LibsqlConnection.java 748 → 734 (under the 740 budget);
LibsqlSavepoint.java 43 → 70.

Probe

Switched lastIndexOf to indexOf in closeTransactionOwnedBySavepoint
(the duplicate-name resolution), one line, reverted:

[ERROR] Tests run: 17, Failures: 1, Errors: 0, Skipped: 0
[ERROR] com.github.magnusp.libsql.jdbc.LibsqlSavepointTest.releasingTheLaterOfTwoSavepointsWithTheSameNameKeepsTheEarlierOneActive:225
Expecting actual:
  ...
but some elements were not expected: "BEGIN", (Close)

The later duplicate then looked outermost, so the release committed and closed
the stream and the next statement re-began. Reverted; LibsqlSavepointTest
17/17 green again.

Verification

Lane	Command	Result
unit	./mvnw -pl libsql-jdbc -am test	398 tests, 0 failures, 0 errors
spotless	./mvnw spotless:check	clean
conformance	./mvnw -pl libsql-conformance -am verify	272 tests, 0 failures, 7 skipped
itests	DOCKER_CONFIG=$PWD/libsql-conformance/src/test/docker-config ./mvnw -pl libsql-itests -am verify	43 tests, 0 failures, 0 errors