[feat][node] Add join stage-1 orchestration and primary test

[TheP2P (thep2p)]

Summary

• Add join_stage1_link_level0, composing the three round-trip primitives from the earlier PRs in this stack: picks the search direction by comparing introducer.id() against this node's own id (Direction::Right when less, Direction::Left when greater, a typed error on collision), locates the search-resolved node and its neighbor, then sends the two GetLinkOp requests concurrently.
• Update the design doc (Section 3.2) to describe both directional branches, replacing an earlier version that only covered introducer.id() < u.id() and, as written, would have silently corrupted the graph's sorted-order invariant when a joining node's id is smaller than its introducer's.
• New primary test (test_join_stage1_link_level0_links_both_sides): the normal two-sided link case, asserting entries land on the correct side of this node's own table.

Part of #90's join stage-1 linking, split into a PR stack: the round-trip primitives (address, span-parentage, predecessor search, neighbor query, link request) are stacked as separate PRs below this one. Two more scenario tests (one-node-graph edge case, no-right-neighbor) plus a fourth (new-leftmost-node, covering the branch this PR's doc fix addresses) follow as small stacked PRs on top of this one.

Test plan

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