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.

internal/web/translation/ja-JP.json

@@ -1384,6 +1384,10 @@
         "testError": "アウトバウンドのテストに失敗しました",
         "testModeTooltip": "TCP: 高速 dial-only プローブ。HTTP: xray を経由した完全リクエスト。",
         "testAll": "すべてテスト",
+        "httpStatus": "HTTPステータス",
+        "breakdownConnect": "プロキシ接続",
+        "breakdownTls": "アウトバウンド経由のTLS",
+        "breakdownTtfb": "最初のバイト",
         "nordvpn": "NordVPN",
         "accessToken": "アクセストークン",
         "country": "国",