v6: confirm cluster endpoint details (gRPC port + extension list)

[Pandys]

Small but blocking for transport defaults:

1. Real gRPC port on the public hosts: 443, or 50051 as in the 5.x default (data.clappform.com:50051)? Per cluster if they differ.
2. Canonical list of the 6 cluster extensions (""/prod, qa, qa-lts, ...) — goes in the docs; later possibly a discoverable registry so users can enumerate valid cluster= values.
3. Confirm every API family follows {service}{-extension}.clappform.com (data, auth, client, notifier) — any stragglers get endpoints= documentation.

[Pandys]

Closing as no longer needed. The endpoint details this issue set out to confirm are settled in the shipped client rather than pending an answer:

• gRPC port: the main cluster defaults to :50051, every other cluster to :443 (commit "fix: default the main cluster's gRPC endpoints to :50051").
• Cluster extensions and the {service}{-extension}.clappform.com convention are derived automatically by DNS-based cluster discovery (_discovery.py), so a caller passes only location= and the extension follows from the CNAME — no hardcoded list to maintain.

This has been verified end to end: the notebook in the examples folder connects with self-discovery (no explicit cluster=) and reaches the right cluster, so the open question is answered in practice. Any future need for an enumerable registry of valid cluster= values can be a fresh, scoped issue.