Connection factories in src/conf/connect.py, client wrappers with
auto-auth (_nessie, _polaris, _s3), S3/network config for notebooks,
custom marimo snippets (connections, charts, queries), and _template.py.
33 new tests, 100% coverage on all new code.
fix#72, fix#73, fix#74, fix#75, fix#76, fix#77