Adds MigrationSqlTransactionClassifier to recognize migration SQL that opens
its own transactions (BEGIN/COMMIT/ROLLBACK) so MigrationRunner can skip
wrapping those files in an outer transaction. StartupMigrationHost now surfaces
a MigrationCategory indicator for runtime-aligned bootstrap. Test harness
extended with an explicit-transaction fixture and execution scenario coverage.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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