self-review 2026-05-24: BM25 scan cap 500→5000 + traversal inference guard

Two improvements:

1. BM25 search corpus coverage (server.el) — raised scan cap from 500 to
   5000 nodes. On the 161K-node graph, 500 was 0.3% coverage; 5000 is 3%.
   engram_scan_nodes_json returns nodes sorted by salience DESC, so ISEs
   (salience 0.3) fall below Knowledge/Memory (0.5–0.8) naturally — the
   effective corpus stays content-dense. Also updated stale comment on the
   ISE route (no longer need high offset; recent-first ordering from May 23).

2. Traversal inference guard (el_runtime.c) — two changes:
   - INFER_CAP reduced 256→32: proactive curiosity runs engram_activate
     every ~30s. At 256 edges/call the soul daemon accumulated 107K edges
     in 23h (5× BFS slowdown). At 32 the rate drops 8×.
   - Edge count guard: skip inference entirely when snap_ec ≥ 40,000.
     At that density most A→C paths are already explicit; marginal inference
     value is low and the O(edge_count²) inner-loop cost is high. Self-limits
     unbounded accumulation across restarts.

engram/dist/engram.c

@@ -130,8 +130,8 @@ el_val_t bm25_search_json(el_val_t query, el_val_t limit) {
   if (scan_limit < 200) {
     scan_limit = 200;
   }
-  if (scan_limit > 500) {
-    scan_limit = 500;
+  if (scan_limit > 5000) {
+    scan_limit = 5000;
   }
   el_val_t nodes_json = engram_scan_nodes_json(scan_limit, 0);
   el_val_t n = json_array_len(nodes_json);