swarm: convergence strategies + failure threshold, hardened El JSON usage

- vote/merge/reduce/collect convergence proven end-to-end; failure threshold
  aborts a swarm below min_success_ratio (integer per-mille) and completes
  when failures are within tolerance, with worker.failed + swarm.aborted
  tracked durably.
- worked around three El runtime/codegen semantics surfaced during the build:
  json_set inserts RAW (use json_set_str for string values); json_set cannot
  update an existing key (vote tallies via list rescanning); json_array_get
  keeps quotes (use json_array_get_string). Also: float division is unreliable
  (swarm uses integer math), and a let-rebind in a deeply nested if/else does
  not propagate outward (accumulators kept at one block level).

test_convergence: 8/8; test_swarm: 12/12.

lang/swarm/ccr.el

@@ -88,7 +88,7 @@ fn ccr_retrieve_scoped(blueprint: String, knowledge_refs: String, input_item: St
     let m: Int = json_array_len(knowledge_refs)
     let i = 0
     while i < m {
-        let ref: String = json_array_get(knowledge_refs, i)
+        let ref: String = json_array_get_string(knowledge_refs, i)
         let hit: String = primitive_attend(ref, 3)
         let acc = acc + "# ref:" + ref + "\n" + hit + "\n"
         let i = i + 1