[test][node] Cover join stage-1 becoming the new leftmost node

[TheP2P (thep2p)]

Summary

• Add test_join_stage1_link_level0_new_leftmost_node_sends_single_link_request: the mirror of the no-right-neighbor test, covering introducer.id() > u.id(), where u becomes the new leftmost node reachable from introducer. This is the scenario the direction-selection fix (in the orchestration PR earlier in this stack) exists to handle correctly, so it's the test that actually proves that fix.
• Two small carried-over fixes caught during final verification against the reference implementation, both one-line: BaseNode::search_by_id's own span was missing its parent: &self.span (the pre-existing synchronous search method, missed when the span-parentage PR was split out earlier in this stack), and Core::search_by_id's doc comment was missing the paragraph explaining why the relay chain's self-terminating fallback is only sound when the caller picks the search direction so the first hop already satisfies it, the doc-side explanation of the same invariant the orchestration PR's direction-selection logic depends on.

This completes #90's required test coverage: normal two-sided link, one-node-graph edge case, and no-known-neighbor (both sides) are now all covered.

Test plan

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo doc --no-deps