v6: Clappform client class — constructor, sub-client wiring, lifecycle

[Pandys]

The single entry point tying transport + auth + generated services together.

• Clappform(cluster, location, *, api_key, credentials=None, endpoints=None, insecure=False, timeout=60.0, retries=..., channel_options=None, transport=None)
• Constructor args ONLY — no config files, no env lookups inside the library
• Sub-clients: cf.data, cf.client, cf.auth, cf.notifier (generated); raw stubs importable from clappform.gen; live channel reachable via cf.channel_for("data")
• Context manager + close(); thread-safe for calls
• Helpful __repr__: cluster, location, auth type, __version__, __proto_version__

Acceptance: the full mock walkthrough in the design examples runs against LocalMock.

[Pandys]

Delivered on Major/6 in PR #55. Ships to main when Major/6 is promoted.