Isolate PoI test-cluster tests from default workspace builds

## 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.

Start by locating the poi-rs Cargo manifest, the test-cluster dependency, and the cluster-dependent tests and helpers. Inspect the Ubuntu CI job, then run the workspace tests and the specified release command with test-cluster-tests enabled. Done means default workspace tests omit test-cluster while CI preserves all cluster, epoch-transition, and multi-epoch coverage.

Written by the indexing model from the issue text.