M6 — Client v1.0 core

[nficano]

Goal

Client v1.0 core.

Scope

• Add src/arcp/_client/client.py (handshake driver via ARCPClient(...).connect(transport), request-id correlation, event tail)
• _client/handles.py (JobHandle.done awaitable property, JobHandle.events() iterator)
• v1.0 end-to-end happy path on MemoryTransport and WebSocketTransport (tests/e2e/)

Spec §

§6.1–§6.3, §6.7, §7.1–§7.4

Risk

L