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.

builtin/core.wren

@@ -29,7 +29,18 @@ class Sequence {
     return acc
   }
 
-  reduce(f) { this[1..-1].reduce(this[0], f) }
+  reduce(f) {
+    var iter = iterate(null)
+    if (!iter) Fiber.abort("Can't reduce an empty sequence.")
+
+    // Seed with the first element.
+    var result = iteratorValue(iter)
+    while (iter = iterate(iter)) {
+      result = f.call(result, iteratorValue(iter))
+    }
+
+    return result
+  }
 
 }