Stack optional column retirement contract on operation bridge

[alexstandiford]

Summary

Adds the reviewed optional named-column retirement contract from PR38 on top of operation bridge PR42. The bridge files remain unchanged.

The contract adds the optional TableColumnRetirementStrategy, its recoverable unsupported-capability exception, and the reflection and compatibility tests. Its documentation keeps the reviewed safety rules for validation before mutation, backend identifier semantics, dependency refusal, unrelated-column preservation, and request deduplication.

Stack

• Base: codex/operation-local-handler-bridge at 4b0b7709e5945a4290b04c2e3562ef919923ed08
• Head: codex/operation-bridge-retirement-convergence
• Source contract: DB PR38 head bab214603c3824f98cb964ec226229bbe468167d

Testing

• composer validate --strict passed.
• vendor/bin/phpunit --configuration phpunit.xml passed: 177 tests, 531 assertions.
• vendor/bin/phpstan analyse lib tests --level=9 passed.
• Changed files pass PHP CS Fixer dry-run.
• git diff --check passed.
• Repository-wide PHP CS Fixer dry-run still reports 46 pre-existing files from the bridge base.

Draft only. Do not merge or release from this PR.