[Node] Implement: periodic backpointer repair sweep #89

Summary

Implement the repair sweep itself: on each RepairSchedule tick, send one CheckNeighborOp per currently-populated table entry, and manage the task's lifecycle.

Details

See docs/protocol/concurrent-insert.md, Sections 6.1 and 6.4, for the sweep algorithm and lifecycle requirements.

Files

• src/node/base_node.rs — sweep loop spawned via tokio::spawn against a child of the node's IrrevocableContext; JoinHandle stored and aborted on shutdown (RAII, no orphaned task).

Acceptance Criteria

• Sweep is unconditional per entry (per Section 6.1) — no local staleness pre-guessing.
• Task is cancelled if the node's parent context is cancelled, and can be cancelled independently.
• Tick interval is abstracted behind a schedule trait, not a literal tokio::time::sleep/interval call in the task body, with a test-controlled implementation enabling deterministic single-tick triggering.
• Test: one tick sends the expected probes for a populated table.
• Test: task aborts cleanly on shutdown with no residual state.

Depends On

• [Network] Add: join/repair Event variants and payload structs #84
• [Node] Add: RepairSchedule trait abstracting repair-tick waits #87

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 <-- YOU ARE HERE
    L2:                   #90  #91  #95  #96
    L2.5 (link round-trip follow-ups; #146 blocks #92): #145  #146
    L2.5 (stage-1 concurrent-join fix; blocked by #90, blocks #97): #148
    L3:                   #92
    L4 (acceptance gate): #97
    L5 (post-gate cleanup; also blocked by #76): #100
  Test-suite cleanup, blocked by #90:
    #147
  Test infrastructure, mock network hub (#62's latency simulation builds on #149's queue):
    #149 (blocked by #90, blocks nothing)
    #62 (no blocker)