feat: replace eager list-based map/where with deferred MapSequence and WhereSequence classes

Introduce MapSequence and WhereSequence subclasses of Sequence that apply transformations lazily during iteration instead of building intermediate lists. Update Sequence.map and Sequence.where to return these deferred wrappers, modify all existing tests to call .list on results, and add new infinite-sequence tests verifying lazy behavior on Fibonacci iterators.

test/core/sequence/map.wren

@@ -0,0 +1,46 @@
+// Infinite iterator demonstrating that Sequence.map is not eager
+class FibIterator {
+  new {
+    _current = 0
+    _next = 1
+  }
+
+  iterate {
+    var sum = _current + _next
+    _current = _next
+    _next = sum
+  }
+
+  value { _current }
+}
+
+class Fib is Sequence {
+  iterate(iterator) {
+    if (iterator == null) return new FibIterator
+    iterator.iterate
+    return iterator
+  }
+
+  iteratorValue(iterator) { iterator.value }
+}
+
+var squareFib = (new Fib).map {|fib| fib * fib }
+var iterator = null
+
+IO.print(squareFib is Sequence) // expect: true
+IO.print(squareFib) // expect: instance of MapSequence
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 0
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 4
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 9