feat: enforce extension manifest + shared registry runtime parity

[eva57gr]

No description provided.

[Lightheartdevs]

additionalProperties: false on the manifest schema will break all existing extension manifests that have undeclared fields. Either bump the schema version so old manifests aren't suddenly invalid, or add a migration script. The importlib-based service-registry sharing also adds fragile path inference at import time.

[Lightheartdevs]

The schema tightening is good work, but additionalProperties: false with many new required fields is a breaking change for existing manifests. Please:

1. Audit all existing manifests in the repo to ensure they comply
2. Add a migration guide or script
3. Consider using additionalProperties: false only in a future version bump