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.

doc/site/core/sequence.markdown

@@ -74,14 +74,13 @@ Creates a [list](list.html) containing all the elements in the sequence.
 
 ### **map**(transformation)
 
-Creates a new list by applying `transformation` to each element in the
-sequence.
+Creates a new sequence that applies the `transformation` to each element in the
+original sequence while it is iterated.
 
-Iterates over the sequence, passing each element to the function
-`transformation`. Generates a new list from the result of each of those calls.
+The `list` method can be used to turn the resulting sequence into a list.
 
     :::dart
-    [1, 2, 3].map {|n| n * 2} // [2, 4, 6].
+    [1, 2, 3].map {|n| n * 2}.list // [2, 4, 6].
 
 ### **reduce**(function)
 
@@ -95,10 +94,13 @@ Similar to above, but uses `seed` for the initial value of the accumulator. If t
 
 ### **where**(predicate)
 
-Produces a new list containing only the elements in the sequence that pass the
-`predicate`.
+Creates a new sequence containing only the elements from the original sequence
+that pass the `predicate`.
 
-Iterates over the sequence, passing each element to the function `predicate`.
-If it returns `true`, adds the element to the result list.
+During iteration, each element in the original sequence is passed to the
+function `predicate`. If it returns `false`, the element is skipped.
 
-    (1..10).where {|n| n % 2 == 1} // [1, 3, 5, 7, 9].
+The `list` method can be used to turn the resulting sequence into a list.
+
+    :::dart
+    (1..10).where {|n| n % 2 == 1}.list // [1, 3, 5, 7, 9].