feat(transaction): compose PDO transactions for v1.3.0

[megyptm]

Summary

• Add TransactionRunnerInterface and PdoTransactionRunner with explicit transaction ownership and composition semantics.
• Keep PDO hidden inside the implementation; consumers depend on run(callable $callback): mixed.
• Make ScopedOrderingManager::moveWithinScope() participate in caller-owned transactions while preserving ordering, locking, timestamps, soft-delete, nullable-scope, and failure behavior.
• Retain OrderingTransactionException as a public, autoloadable, deprecated 1.x compatibility class without using it in the new transaction flow.
• Include unit, regression, and Real MySQL integration coverage.

Verification

• Unit: PASS — 241 tests, 379 assertions
• Regression: PASS — 37 tests, 515 assertions
• PHPStan, syntax, code style, Composer validation, platform checks, and audit: PASS
• Real MySQL 8.4.10 integration and full suite: PASS on PHP 8.2 and PHP 8.5
• Lowest dependencies, PHP 8.2–8.5 matrix, residue verification, workflow lint, and CI Gate: PASS
• Final CI run: https://github.com/Maatify/persistence/actions/runs/34453695327

Release readiness

• CHANGELOG.md is prepared for v1.3.0 dated 2026-09-10, with correct compare links.
• README and composer.json describe Ordering, Transactions, and Pagination.
• Historical capability documentation is aligned with the released v1.2.0 Ordering and v1.1.0 Pagination capabilities.
• No tag or GitHub Release has been created.
• After this PR is merged, only creating the v1.3.0 tag and GitHub Release remains.