docs: add concise ACP HCP etcd design (no ManagedEtcd CRD)

[jiazhiguang]

Add docs/design/hcp-etcd-design.md, a trimmed, human-readable design for managing etcd under ACP HCP. It carries over the goals of the original acp-hcp-managed-etcd-crd-design.md but corrects them against the current codebase and drops the high-level ManagedEtcd CRD.

Key decisions:

• No new high-level CRD. A codebase audit showed the low-level EtcdCluster controller already drives etcd membership directly (MemberList/Add/Remove/ PromoteLearner via internal/etcdutils), so the capabilities the original design assigned to ManagedEtcd (status aggregation, scheduling protection, single-member recovery) are generic etcd concerns. They belong on EtcdCluster and are potentially upstreamable, not in a separate CRD.

• Two tracks instead of a wrapper CRD:

  • Track 1 (generic, upstreamable): enrich EtcdCluster. Populate the currently-empty EtcdClusterStatus (members/leader/health/conditions) from the data already gathered in-process; extend podTemplate with nodeSelector/tolerations/affinity/topologySpreadConstraints; add a PDB and a -client Service; add HyperShift-style single-member recovery (Job-based, gated on quorum + gracePeriod).
  • Track 2 (ACP-specific, optional): publish the Kamaji DataStore. Since track 1 makes the client endpoint and -client-tls secret stable and predictable, "who creates the DataStore" is a replaceable integration point with three options: (A) manual/declarative as a zero-code fallback, (B) a watcher in the Kamaji control-plane provider (preferred — keeps the Kamaji dependency out of etcd-operator), (C) an opt-in reconciler inside the operator. None of them add Kamaji fields to EtcdCluster.spec.

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