Stack named-column retirement on coordinated MySQL operations

[alexstandiford]

What changed

This draft restacks the named-column retirement implementation and its unit and real MySQL contracts on the corrected TableExists layer. It also retains the narrow PDO type changes needed for the full PHPStan run.

Stack

• base: PR Read table existence from query result rows #30 at 96ab539c3e44fb721c430c717257e9bb462712a8
• prerequisite DB contract: draft PR Stack optional column retirement contract on operation bridge db#43 at 76d7161702a4368748b6865cd05001dce0b157cc
• the exact DB head is pinned in composer.json and composer.lock as a temporary review dependency

Proof

Against isolated MySQL 8.4.11:

• retirement unit contracts: 27 tests, 32 assertions
• retirement real-driver contracts: 15 tests, 38 assertions, no skips
• full package: 841 tests, 5,246 assertions
• the two full-suite skips are the existing MySQL 5.7-only contracts
• PHPStan level 5 passes for lib and tests
• repository spellcheck passes
• changed retirement files pass the repository formatter
• composer validate reports only the expected temporary commit-ref review warning

No merge, release, tag, or deployment is included.