[test][node] Cover the one-node-graph join stage-1 edge case

[TheP2P (thep2p)]

Summary

• Add test_join_stage1_link_level0_one_node_graph_edge_case: introducer's own reply names introducer itself as s, the existing search_by_id fallback exercised unmodified. join_stage1_link_level0 needs no special-casing and completes exactly as it would for a distinct s.
• Required acceptance-criteria coverage for [Node] Implement: level-0 join linking (Phase 1) #90 (issue [Node] Implement: level-0 join linking (Phase 1) #90 explicitly lists this as one of the three required test scenarios). No production code change: the orchestration this exercises already shipped in the previous PR in this stack.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test