bg: soften sidecar readiness gate for non-MasterDnsVPN sidecars

awaitExternalProcessesReady previously threw IOException (hard VPN-start
failure) for any sidecar that didn't bind within maxOf(1s, connectionTestTimeout).
MasterDnsVPN genuinely needs that gate (it crashed if dialed before binding),
but Mieru/Naive/TrojanGo/Hysteria were historically fire-and-forget with
first-dial retry. Now only MasterDnsVPN throws on timeout; the others log a
warning and continue. (Greptile, PR #18 follow-up)

Author: hawkff

app/src/main/java/io/nekohasekai/sagernet/bg/proto/BoxInstance.kt

@@ -294,7 +294,16 @@ abstract class BoxInstance(
                 if (pending.isNotEmpty()) delay(50)
             }
             if (pending.isNotEmpty()) {
-                throw IOException("sidecar listener not ready on port(s): ${pending.joinToString()}")
+                // MasterDnsVPN must have its listener up before the first dial (it crashed
+                // otherwise), so a timeout there is fatal. Other sidecars (Mieru/Naïve/
+                // TrojanGo/Hysteria) were historically fire-and-forget: the first sing-box
+                // dial retries, so a slow bind shouldn't hard-fail VPN start — log and continue.
+                val message = "sidecar listener not ready on port(s): ${pending.joinToString()}"
+                if (hasMasterDnsVpn) {
+                    throw IOException(message)
+                } else {
+                    Logs.w("$message; continuing (sing-box will retry the connection)")
+                }
             }
         }
     }