feat: implement single-argument reduce with manual iteration and empty-sequence abort

Replace the previous slice-based single-argument reduce implementation with an explicit iterator loop that seeds from the first element. This avoids the out-of-bounds range error on single-element lists and adds a Fiber.abort for empty sequences. Update the test to verify correct behavior for single-element and empty sequences.

src/wren_core.c

@@ -72,7 +72,18 @@ static const char* libSource =
 "    return acc\n"
 "  }\n"
 "\n"
-"  reduce(f) { this[1..-1].reduce(this[0], f) }\n"
+"  reduce(f) {\n"
+"    var iter = iterate(null)\n"
+"    if (!iter) Fiber.abort(\"Can't reduce an empty sequence.\")\n"
+"\n"
+"    // Seed with the first element.\n"
+"    var result = iteratorValue(iter)\n"
+"    while (iter = iterate(iter)) {\n"
+"      result = f.call(result, iteratorValue(iter))\n"
+"    }\n"
+"\n"
+"    return result\n"
+"  }\n"
 "\n"
 "}\n"
 "\n"