Isolate PoI test-cluster tests from default workspace builds

[itsyaasir]

Problem

test-cluster is an unconditional poi-rs dev dependency. Default workspace test builds therefore compile much of the IOTA node stack on every platform, even when running only lightweight tests.

Proposed change

• Make test-cluster optional behind a test-cluster-tests feature.
• Gate only cluster-dependent tests and helpers; keep offline tests enabled by default.
• Run cargo test -p poi-rs --features test-cluster-tests --release -- --test-threads=1 once on Ubuntu CI.

Acceptance criteria

• cargo test --workspace does not compile test-cluster.
• CI still runs all PoI test-cluster tests single-threaded.
• Existing epoch-transition and multi-epoch coverage is preserved.