feat(dns): add intelligent DNS Benchmark, Auto-Tuner & clean defaults

- Implement ucode module dns/benchmark.uc:
  * Probes major DNS servers (Yandex, Cloudflare, Google, Quad9, AdGuard, Comss, Mullvad, Control D) across UDP 53 and DoH.
  * Measures latency, packet loss, and detects ISP DNS poisoning/hijacking.
  * Intelligent recommendation engine computes optimal primary DNS (prefers encrypted DoH if fast), bootstrap plain UDP, fallback plain UDP from independent provider, and upstream mode (sequential/parallel).
  * Supports CLI execution (tachyon dns_benchmark, tachyon dns_autotune --apply) and background async worker mode for LuCI UI.
- LuCI Web UI modal (renderDnsBenchmarkModal.ts):
  * Real-time progress bar, responsive results table with latency rating badges.
  * Recommendation summary card with one-click 'Apply Recommended DNS' action.
  * Added '⚡ Auto-Tune & Benchmark DNS' button in Settings -> DNS tab.
- Added comprehensive unit tests in vitest (538/538 passed) and tests/dns_benchmark.sh.
- Added Russian PO translations.

luci-app-tachyon/htdocs/luci-static/resources/view/tachyon/settings.js

@@ -1326,6 +1326,23 @@ function createSettingsContent(section, capabilities) {
 
   // ── DNS Settings ────────────────────────────────────────────────────────
 
+  const dnsBenchmarkBtn = section.taboption(
+    "dns",
+    form.Button,
+    "_dns_benchmark_btn",
+    _("DNS Auto-Tuner & Benchmark"),
+    _(
+      "Automatically benchmark latency, packet loss, and anti-censorship reliability of major DNS servers (UDP/DoH) from this router to find and apply the optimal configuration.",
+    ),
+  );
+  dnsBenchmarkBtn.inputtitle = "⚡ " + _("Auto-Tune & Benchmark DNS");
+  dnsBenchmarkBtn.inputstyle = "action important";
+  dnsBenchmarkBtn.onclick = function () {
+    if (main && typeof main.renderDnsBenchmarkModal === "function") {
+      main.renderDnsBenchmarkModal();
+    }
+  };
+
   settingsDnsDynamicState.dnsType =
     uci.get(UCI_PACKAGE, "settings", "dns_type") || "udp";