Add multilingual exercise code (#1959)

Replace the exercise pages' Python-only snippets with source-backed implementations for the 13 visible programming languages, and localize reader-facing comments by site language. Zig remains out of scope.

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ru/codes/dart/chapter_computational_complexity/complexity_exercises.dart

@@ -0,0 +1,63 @@
+/**
+ * File: complexity_exercises.dart
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+/* Итеративное суммирование */
+int sumIter(int n) {
+  int res = 0;
+  for (int i = 1; i <= n; i++) {
+    res += i;
+  }
+  return res;
+}
+
+/* Рекурсивное суммирование */
+int sumRecur(int n) {
+  if (n == 1) {
+    return 1;
+  }
+  return n + sumRecur(n - 1);
+}
+
+/* Цикл линейной сложности */
+int linearLoop(int n) {
+  int res = 0;
+  for (int i = 0; i < n; i++) {
+    res += i;
+  }
+  return res;
+}
+
+/* Цикл квадратичной сложности */
+int quadraticLoop(int n) {
+  int res = 0;
+  for (int i = 0; i < n; i++) {
+    for (int j = i; j < n; j++) {
+      res += j;
+    }
+  }
+  return res;
+}
+
+/* Цикл логарифмической сложности */
+int logarithmicLoop(int n) {
+  while (n > 1) {
+    n ~/= 2;
+  }
+  return n;
+}
+
+void main() {
+  if (sumIter(1) != 1 ||
+      sumRecur(1) != 1 ||
+      sumIter(4) != 10 ||
+      sumRecur(4) != 10 ||
+      linearLoop(4) != 6 ||
+      quadraticLoop(4) != 20 ||
+      logarithmicLoop(4) != 1 ||
+      logarithmicLoop(5) != 1) {
+    throw StateError('complexity exercise check failed');
+  }
+}