Follow-ups from the rpc.rs split (#4110): SDK=stale surface, broken doc link, stale citations#4115

Found while reviewing #4110 (T4-F1). None block it; scored per AGENTS.md (F x S).

1. rpc.rs drops out of the SDK=stale surface (F 2 x S 3 = 6). core/scripts/artifacts.py:177 counts a file as SDK surface when it contains ts_rs or TS. After the split rpc.rs has neither, yet it still defines served_capabilities(), from which capabilities.d.ts/.js/.mjs are built. A later edit to that function alone could leave the opt-in local SDK=stale mode on a stale SDK. CI is unaffected. Fix: add crates/ployz-core/src/rpc.rs to the sdk surface tuple.
2. Broken intra-doc link: Departure's doc in core/crates/ployz-core/src/rpc/error.rs links [HealthVerdict], which is not in scope there. Use [HealthVerdict](crate::HealthVerdict). The ts-rs output is unchanged.
3. Stale wording: the #[expect(clippy::result_large_err)] reason in core/crates/ployzd/src/docker/volume.rs still says the type is in the contract crate.
4. Stale cites: docs/agents/design/cluster-ingress/engine-pointers.md:48 (and the docs/agents/plans/** line cites) point at rpc.rs for DTOs that now live in rpc/ingress.rs and its siblings.
5. ployz_core::rpc::CreateVolumeRequest no longer resolves (the type moved to domain/volume.rs; ployz_core::CreateVolumeRequest is unchanged and is the only path used in the tree).