[codex] Use plain IN for single WordPress identity fields

[alexstandiford]

Summary

• Generate plain single-column IN clauses for single identity-field lookups in the WordPress clause builder.
• Preserve row-constructor IN SQL for true compound identity fields.
• Add unit coverage for single identity, multiple single-field identities, and compound identities.

Why

WordPress fetches records such as posts with a direct primary-key predicate (WHERE ID = %d LIMIT 1). PHPNomad's WordPress integration was generating row-constructor SQL even for a single identity field, e.g. (alias.id) IN ((%s)). This keeps the PHPNomad API unchanged while making the WordPress integration emit the simpler primary-key lookup shape WordPress itself uses for single-column identities.

Validation

• vendor/bin/phpunit --filter ClauseBuilderTest --testdox
• vendor/bin/phpunit --testdox

[alexstandiford]

Closing this because it only makes the single-field IN query shape more WordPress-like. It is not proven to fix the intermittent create/readback failure and should not be represented as the root-cause fix.