feat(outbound): batched connection tester with direct timed HTTP probes

Replace the per-outbound burstObservatory polling (one temp xray spawn +
up to 15s of /debug/vars polling per outbound, serialised) with one
shared temp xray instance per batch: every tested outbound gets its own
loopback SOCKS inbound plus an inboundTag->outboundTag routing rule, and
the panel times a real HTTP request through each one in parallel. The
probe returns as soon as the response lands and records the HTTP status
plus an httptrace breakdown (proxy connect / TLS via outbound / first
byte) shown in the result popover.

New POST /panel/api/xray/testOutbounds endpoint (array in, results in
input order, max 50); the legacy /testOutbound endpoint now delegates to
the same engine. Test All chunks HTTP probes 16 per request, and a batch
whose shared process never comes up (one structurally-broken outbound
poisons the config) retries each item in an isolated instance so the
broken outbound reports xray's real error while the rest still test.

frontend/src/pages/api-docs/endpoints.ts

@@ -1063,6 +1063,17 @@ export const sections: readonly Section[] = [
         ],
         body: 'outbound={"protocol":"freedom","settings":{}}&mode=tcp',
       },
+      {
+        method: 'POST',
+        path: '/panel/api/xray/testOutbounds',
+        summary: 'Test a batch of outbounds (max 50) through one shared temp xray instance. Returns an array of results in input order, each with the outbound tag, delay, HTTP status and a connect/TLS/TTFB timing breakdown.',
+        params: [
+          { name: 'outbounds', in: 'body (form)', type: 'string', desc: 'JSON array of outbound configs to test (required).' },
+          { name: 'allOutbounds', in: 'body (form)', type: 'string', desc: 'JSON array of all outbounds — used to resolve dialerProxy chains.' },
+          { name: 'mode', in: 'body (form)', type: 'string', desc: '"tcp" for fast dial-only probes (UDP-transport outbounds are still probed over HTTP). Default/empty routes a real HTTP request through each outbound.' },
+        ],
+        body: 'outbounds=[{"tag":"direct","protocol":"freedom","settings":{}}]&mode=http',
+      },
       {
         method: 'POST',
         path: '/panel/api/xray/balancerStatus',