Correct datastore static contracts

[alexstandiford]

Summary

• declare the Event interface package used by public datastore event classes
• repair malformed datastore generics and propagate model types
• document actual condition, identity, event payload, and model adapter shapes
• preserve optional AND defaults and string or tuple columns

Verification

• composer install --no-interaction --no-progress
• composer validate --strict --no-check-publish
• composer audit reports no advisories
• PHPUnit: 1 test, 1 assertion
• configured PHPStan level 9: no errors
• locked dependencies remain compatible with PHP 7.4.33

This draft is stacked on #11. The runtime dependency resolves to phpnomad/event 1.0.1 at b418d8fc90af18b7cb37e118795f9dd92ffeb538. PHP source changes are documentation only and preserve native signatures and behavior.

[alexstandiford]

Superseded by #11. The prerequisite and contract work now lives in one independently green draft. Both branches have the same final tree, f32936e7eff515c03da91faa3911cf0277575680.