Improve failover controls and preserve manual switching state

.tmp-tests/harbor-startup-recovery-xqwmBz/bin/sing-box

@@ -0,0 +1,11 @@
+#!/usr/bin/env node
+const fs = require('node:fs');
+if (process.argv[2] === 'check') process.exit(0);
+if (process.argv[2] === 'version') {
+  console.log('sing-box version 1.13.18');
+  process.exit(0);
+}
+if (process.argv[2] === 'run') {
+  fs.appendFileSync(process.env.HARBOR_TEST_RUN_MARKER, 'run\n');
+  setInterval(() => {}, 60_000);
+}

.tmp-tests/harbor-startup-recovery-xqwmBz/sing-box-config.json

@@ -0,0 +1 @@
+{"outbounds":[{"type":"vless","tag":"server-a","server":"state.example","server_port":443},{"type":"direct","tag":"direct"}],"route":{"final":"server-a"}}

.tmp-tests/harbor-startup-recovery-xqwmBz/state.json

@@ -0,0 +1 @@
+{"schemaVersion":4,"revision":2,"subscriptionUrl":"https://state.example/subscription-a","servers":[{"id":"server-a","label":"State server","host":"state.example","port":443,"protocol":"vless"}],"selectedServerId":"server-a","appliedServerId":"server-a","connectionDesired":"running"}

.tmp-tests/harbor-startup-recovery-xqwmBz/subscription-cache.json

@@ -0,0 +1 @@
+{"url":"https://cache.example/subscription-b","config":{"outbounds":[{"id":"server-b","label":"Cache server","host":"cache.example","port":8443,"protocol":"vless","type":"vless","tag":"Cache server","server":"cache.example","server_port":8443}]},"servers":[{"id":"server-b","label":"Cache server","host":"cache.example","port":8443,"protocol":"vless","type":"vless","tag":"Cache server","server":"cache.example","server_port":8443}]}