[Test] Add: deterministic concurrent-insert repair regression test

[thep2p]

Summary

The concurrent-insert epic's acceptance test: reproduce the exact 4-node adversarial interleaving that leaves two stale backpointers, then verify one repair round heals it. Setup and message ordering: docs/protocol/concurrent-insert.md, Section 7.

Scope

• New sibling *_test.rs alongside the join/repair logic (this crate's colocated test convention).
• README.md — update the roadmap to reflect join + repair as implemented.

Acceptance Criteria

• Direct sequential calls through the mock network (synchronous/recursive dispatch), not std::thread::spawn — no timing assumptions.
• Assertion 1: the transient stale state (B.right/C.left pointing past each other) is reachable under the exact ordering in Section 7.
• Assertion 2: one repair round heals it into the fully-reciprocal A–B–C–D chain. Per Section 7, prefer exercising the real spawned repair task via the test RepairSchedule, firing one tick and awaiting completion under a bounded timeout; a direct call to the CheckNeighborOp handler is an acceptable fallback.
• README.md roadmap entry for join/repair moved from "Next" to "Implemented".

Depends On

• [Network] Add: join/repair Event variants and payload structs #84 — join/repair message catalogue
• [Core] Add try_link: atomic accept-or-forward slot decision #85 — try_link primitive
• [Core] Add try_relink: single-lock repair primitive for LookupTable #93 — try_relink primitive
• [Node] Add: prefix-match and max-level queries to Core #86 — prefix-match and max-level queries
• [Node] Add: RepairSchedule trait abstracting repair-tick waits #87 — RepairSchedule abstraction
• [Node] Add: link/relink-decision handlers to Core #94 — link/relink-decision handlers
• [Node] Generalize request correlation: add Phase 0 join bootstrap #88 — request correlation and join bootstrap
• [Node] Implement: periodic backpointer repair sweep #89 — periodic backpointer repair sweep
• [Node] Implement: level-0 join linking (Phase 1) #90 — Stage-1 level-0 join linking
• [Node] Wire: max-level, neighbor, and set-link event handlers #91 — max-level/neighbor/set-link event handlers
• [Node] Wire: GetLinkOp and BuddyOp event handlers #95 — GetLinkOp and BuddyOp event handlers
• [Node] Wire: CheckNeighborOp repair handler #96 — CheckNeighborOp repair handler
• [Node] Implement: Stage-2 join level climbing #92 — Stage-2 join level climbing

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  #91  #95  #96
    L3:                   #92
    L4 (acceptance gate): #97 <-- YOU ARE HERE