feat(runtime): own same-root class state

[cukas]

Summary

• own restricted same-root class allocation, scalar field initialization, constructor assignment, and direct field access in the effect machine
• snapshot class registry and descriptor metadata so async suspension and post-link mutation cannot change admitted behavior
• preserve legacy compatibility for methods, inheritance, helpers, imports, and non-root class behavior

Validation

• pnpm --filter @kernlang/core test
• pnpm fitness:kern-5
• full-roster Agon review (claude,codex,agy), with verified findings fixed and targeted regression gates rerun