perf: accept thread + dial backoff; join/peers.json (0.9.23)

Run inbound peer handling on its own thread so outbound dials cannot starve
:9100. Failed peers cool down exponentially. Also land --join (member seat)
and realm/peers.json seed discovery for Tailscale meshes.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/discover.c

@@ -250,6 +250,60 @@ static void ingest_local(cst_discover_t *d) {
     remember(d, &list[i]);
 }
 
+/* Optional realm/peers.json for meshes without multicast (e.g. Tailscale):
+ * {"peers":[{"host":"100.x.y.z","port":9100,"iid":"seat-a","name":"a"}]} */
+static void ingest_seed_peers(cst_discover_t *d) {
+  char dir[OS_PATH_MAX], path[OS_PATH_MAX];
+  if (cst_realm_dir(dir, sizeof(dir)) != 0)
+    return;
+  if (os_path_join(path, sizeof(path), dir, "peers.json") != 0)
+    return;
+  FILE *f = fopen(path, "rb");
+  if (!f)
+    return;
+  cst_buf_t b = {0};
+  char chunk[4096];
+  size_t n;
+  while ((n = fread(chunk, 1, sizeof(chunk), f)) && b.len < 64 * 1024)
+    cst_buf_append(&b, chunk, n);
+  fclose(f);
+  cJSON *root = cJSON_Parse(b.buf ? b.buf : "{}");
+  cst_buf_free(&b);
+  if (!root)
+    return;
+  cJSON *arr = cJSON_GetObjectItemCaseSensitive(root, "peers");
+  if (!cJSON_IsArray(arr)) {
+    cJSON_Delete(root);
+    return;
+  }
+  const cJSON *it;
+  cJSON_ArrayForEach(it, arr) {
+    const cJSON *host = cJSON_GetObjectItemCaseSensitive(it, "host");
+    const cJSON *port = cJSON_GetObjectItemCaseSensitive(it, "port");
+    const cJSON *iid = cJSON_GetObjectItemCaseSensitive(it, "iid");
+    const cJSON *name = cJSON_GetObjectItemCaseSensitive(it, "name");
+    const cJSON *rid = cJSON_GetObjectItemCaseSensitive(it, "rid");
+    if (!cJSON_IsString(host) || !host->valuestring[0] || !cJSON_IsNumber(port) ||
+        port->valueint <= 0 || !cJSON_IsString(iid) || !iid->valuestring[0])
+      continue;
+    if (cJSON_IsString(rid) && rid->valuestring[0] &&
+        strcmp(rid->valuestring, d->realm_fp) != 0)
+      continue;
+    cst_peer_info_t p;
+    memset(&p, 0, sizeof(p));
+    snprintf(p.realm_fp, sizeof(p.realm_fp), "%s", d->realm_fp);
+    snprintf(p.instance_id, sizeof(p.instance_id), "%s", iid->valuestring);
+    snprintf(p.host, sizeof(p.host), "%s", host->valuestring);
+    if (cJSON_IsString(name) && name->valuestring[0])
+      snprintf(p.name, sizeof(p.name), "%s", name->valuestring);
+    else
+      snprintf(p.name, sizeof(p.name), "%s", iid->valuestring);
+    p.port = port->valueint;
+    remember(d, &p);
+  }
+  cJSON_Delete(root);
+}
+
 static int send_beacon(cst_discover_t *d) {
   if (d->sock == DISC_BAD)
     return -1;
@@ -310,6 +364,7 @@ static void *discover_main(void *arg) {
         (void)cst_discover_local_put(d->realm_fp, d->instance_id, d->name,
                                      d->peer_port);
       ingest_local(d);
+      ingest_seed_peers(d);
     }
     ticks++;
     if (d->sock == DISC_BAD) {