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/worktrack.el

@@ -24,6 +24,14 @@
 // Depends on: el_runtime.c builtins (fs_*, http_post, env, json_*, uuid_v4,
 // now_millis, str_*). No El-module concat dependencies of its own.
 
+// ── JSON helper ──────────────────────────────────────────────────────────────
+// json_set inserts its value as a RAW JSON fragment (objects/arrays/numbers).
+// json_set_str sets a plain STRING value, correctly quoted and escaped. Use
+// json_set for nested JSON, json_set_str for strings.
+fn json_set_str(j: String, key: String, val: String) -> String {
+    return json_set(j, key, "\"" + json_escape_string(val) + "\"")
+}
+
 // ── Journal location ─────────────────────────────────────────────────────────
 
 // worktrack_dir — directory holding the swarm journals.
@@ -112,7 +120,7 @@ fn worktrack_mirror_engram(rec: String, corr_id: String, kind: String, subject:
     let body_kv = el_list_append(body_kv, "0.5")
     let body: String = json_build_object(body_kv)
     let key: String = env("ENGRAM_API_KEY")
-    let body2: String = json_set(body, "_auth", key)
+    let body2: String = json_set_str(body, "_auth", key)
     let resp: String = http_post(url + "/api/node", body2)
     return true
 }