runtime: engram_edges_json — read edges without a whole-graph file round trip

/api/graph/edges answered a read query by calling engram_save() to serialize
the ENTIRE graph to disk (128 MB) and then fs_read-ing it back. Two defects in
one line, and both bit production on 2026-08-15:

  1. The path it wrote was ~/.neuron/engram/snapshot.json — the engram
     server's CANONICAL store. A READ route overwriting the persistence
     owner's canonical file. This defect had been fixed once (export moved to
     a scratch path); it came back when the hand-written dispatch block was
     replaced by @route dispatch and the unfixed copy is the one that
     survived the merge.
  2. Cost: a full snapshot write, a 128 MB read, and a parse of the whole
     graph, per request, to return a bounded slice.

Calling it tonight overwrote the canonical snapshot and immediately preceded
an engram crash loop.

engram_edges_json(limit, offset) is the builtin that route's own TODO asked
for ("Future: add an engram_edges_json() builtin and drop the file round trip
entirely"). It walks g->edges directly and emits every persisted field.

limit <= 0 defaults to 1000, not unbounded: this is the endpoint that fell
over, and an unbounded default would preserve the failure mode under a new
name. Callers page explicitly.

Registered in codegen.el's builtin_arity (both plain and __ spellings) and
wrapped in el_seed.c per the project's C-builtin recipe.

lang/runtime/el_runtime.c

@@ -18329,3 +18329,47 @@ el_val_t engram_recall_json(el_val_t query, el_val_t limit) {
 el_val_t cgi_principal(void) { return EL_STR(_el_cgi_principal ? _el_cgi_principal : ""); }
 el_val_t cgi_network(void)   { return EL_STR(_el_cgi_network   ? _el_cgi_network   : ""); }
 el_val_t cgi_engram(void)    { return EL_STR(_el_cgi_engram    ? _el_cgi_engram    : ""); }
+
+/* engram_edges_json(limit, offset) — emit edges straight from the store.
+ *
+ * Replaces a serialize-and-reread round trip that took production down on
+ * 2026-08-15: /api/graph/edges called engram_save() to write the ENTIRE graph
+ * to disk (128 MB) and then fs_read it back, just to answer a read query for
+ * edges. One debug request cost a full snapshot write, a 128 MB read, and the
+ * peak memory to hold it — on top of being O(whole graph) for a bounded slice.
+ * The route's own comment had already named the fix: "Future: add an
+ * engram_edges_json() builtin and drop the file round trip entirely."
+ *
+ * limit <= 0 defaults to 1000 rather than unbounded: this is the endpoint that
+ * fell over, and an unbounded default would preserve the failure mode under a
+ * different name. Pass an explicit limit to page.
+ */
+el_val_t engram_edges_json(el_val_t limit, el_val_t offset) {
+    EngramStore* g = engram_get();
+    int64_t lim = (int64_t)limit;  if (lim <= 0) lim = 1000;
+    int64_t off = (int64_t)offset; if (off < 0)  off = 0;
+
+    JsonBuf b; jb_init(&b);
+    jb_putc(&b, '[');
+    int64_t emitted = 0;
+    char t[192];
+    for (int64_t i = off; i < g->edge_count && emitted < lim; i++) {
+        EngramEdge* e = &g->edges[i];
+        if (emitted > 0) jb_putc(&b, ',');
+        jb_puts(&b, "{\"id\":");        jb_emit_escaped(&b, e->id       ? e->id       : "");
+        jb_puts(&b, ",\"from_id\":");   jb_emit_escaped(&b, e->from_id  ? e->from_id  : "");
+        jb_puts(&b, ",\"to_id\":");     jb_emit_escaped(&b, e->to_id    ? e->to_id    : "");
+        jb_puts(&b, ",\"relation\":");  jb_emit_escaped(&b, e->relation ? e->relation : "");
+        snprintf(t, sizeof t,
+                 ",\"weight\":%.6g,\"hebb\":%.6g,\"confidence\":%.6g,"
+                 "\"created_at\":%lld,\"updated_at\":%lld,\"last_fired\":%lld,"
+                 "\"inhibitory\":%d,\"layer_id\":%u}",
+                 e->weight, e->hebb, e->confidence,
+                 (long long)e->created_at, (long long)e->updated_at,
+                 (long long)e->last_fired, e->inhibitory, (unsigned)e->layer_id);
+        jb_puts(&b, t);
+        emitted++;
+    }
+    jb_putc(&b, ']');
+    return el_wrap_str(b.buf);
+}