Unify statement result attachment and remove the closed=false window #211

Goal

Unify statement result attachment and remove the transient closed = false window. assignResultSet and executeResult are near copies, and closeResultSet flips closed and closing so a nested close can pass the closed check.

Depends on

#195 for the close-path pin.

Pin

LibsqlStatementLifecycleTest covers post-close execute returning a closed result set, close-on-completion, and the requested-result-set error. LibsqlStatementConcurrencyTest covers execute against close. The xerial StatementTest, PrepStmtTest, and MetadataLeakTest cover the same surface.

Scope

• Add one attachResult(...) used by both paths, with a discardCurrentResult() for the closed branch.
• Add LibsqlResultSet.closeDetached() that closes without calling back into the statement.
• Replace the flag flip in closeResultSet (329 to 342).
• Share newQueryResult(...) between LibsqlStatement.executeQuery and LibsqlPreparedStatement.executeQuery.

Preserve

• Post-close execute, executeQuery, executeUpdate, and getConnection keep working and return a closed result set.
• A second result set on the same statement throws "ResultSet already requested".
• closeOnCompletion and isCloseOnCompletion keep the lower-case "statement is closed" message.
• Metadata result sets never use close-on-completion.
• updateCount is -1 when a result set exists and the affected row count otherwise.

Acceptance

• closed is never written outside close() and closeDetached, and no method temporarily reopens the statement.
• LibsqlStatement.java is under 650 lines.
• ./mvnw -pl libsql-jdbc -am test green with no assertion edited.
• ./mvnw -pl libsql-itests -am verify green with DOCKER_CONFIG set per AGENTS.md section 4.