feat: add Random.shuffle() method using Fisher-Yates algorithm

Implement in-place shuffle for lists on the Random class, using the Fisher-Yates algorithm to ensure uniform permutation distribution. Includes documentation with usage examples, Wren source implementation, and a test suite covering empty lists, single-element lists, and verification that all 24 permutations of a 4-element list are reachable.

src/optional/wren_opt_random.wren

@@ -46,4 +46,16 @@ foreign class Random {
   foreign int()
   int(end) { (float() * end).floor }
   int(start, end) { (float() * (end - start)).floor + start }
+
+  shuffle(list) {
+    if (list.isEmpty) return
+
+    // Fisher-Yates shuffle.
+    for (i in 0...list.count - 1) {
+      var from = int(i, list.count)
+      var temp = list[from]
+      list[from] = list[i]
+      list[i] = temp
+    }
+  }
 }

src/optional/wren_opt_random.wren.inc

@@ -48,4 +48,16 @@ static const char* randomModuleSource =
 "  foreign int()\n"
 "  int(end) { (float() * end).floor }\n"
 "  int(start, end) { (float() * (end - start)).floor + start }\n"
+"\n"
+"  shuffle(list) {\n"
+"    if (list.isEmpty) return\n"
+"\n"
+"    // Fisher-Yates shuffle.\n"
+"    for (i in 0...list.count - 1) {\n"
+"      var from = int(i, list.count)\n"
+"      var temp = list[from]\n"
+"      list[from] = list[i]\n"
+"      list[i] = temp\n"
+"    }\n"
+"  }\n"
 "}\n";