feat: add context engine registration adapter

[offendingcommit]

Summary

• add one singular context_engine adapter to register_plugin
• validate the real Hermes ContextEngine contract before host mutation
• record the declared engine in stable registration receipts
• test both deployed Hermes 9b117cc709 and upstream-main drift lanes

Validation

• 118 passed, 12 skipped
• exact-host context contract passed
• package build passed