[Node] Implement: level-0 join linking (Phase 1)

[thep2p]

Summary

Implement Phase 1 of join: locate the predecessor via the existing search machinery, query its right neighbor, and send the two independent GetLinkOp requests that establish u's level-0 neighbors.

Algorithm detail, including the asymmetry note in step 7 (the s-chain and z-chain resolve u's two sides independently — never each other's), is specified in docs/protocol/concurrent-insert.md, Section 3.2.

Files

• src/node/base_node.rs — Phase 1 orchestration, built on the correlation infrastructure from [Node] Generalize request correlation: add Phase 0 join bootstrap #88.

Acceptance Criteria

• Reuses SearchByIdRequest/SearchByIdResponse unmodified for the predecessor search.
• Handles the z = None case (no right neighbor known) without blocking.
• Replies are applied to u's own table via try_link ([Core] Add try_link: atomic accept-or-forward slot decision #85) — no second write path.
• Tests cover:
  • Normal two-sided link.
  • One-node-graph edge case (search returns the introducer itself).
  • z = None at query time.

Dependencies

Depends on: #84, #85, #88.

Dependency graph (unblocked first):
EPIC #81 -- local search & join protocol
  #67 [DONE] search_by_id            #71 [OPEN] search_by_mem_vec (independent)
  #74 [OPEN] search_by_id timeout fix   #76 [OPEN] delete/leave
  Concurrent join + repair (Algorithm 2 + Algorithm 8; supersedes closed #66, #77):
    L0 (no deps):         #84  #85  #93  #86  #87
    L1:                   #94  #88  #89
    L2:                   #90 <-- YOU ARE HERE  #91  #95  #96
    L3:                   #92
    L4 (acceptance gate): #97