refactor: simplify iterator.Try and remove unnecessary backtracking

Simplify Try to save/restore the index directly instead of
copying and syncing the entire iterator. Remove the now-unused
Copy and Sync methods.

Rewrite ScanRune and ParseRawToken as peek-then-advance so they
no longer need Try at all. Remove redundant Try wrappers from
parse functions that are already disambiguated by their callers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/iterator/iterator.go

@@ -19,18 +19,6 @@ func (i Iterator[T]) Index() int {
 	return i.index
 }
 
-// Copy returns a identical clone of the iterator. The underlying data structure
-// is not cloned.
-func (i Iterator[T]) Copy() *Iterator[T] {
-	return &Iterator[T]{items: i.items, index: i.index}
-}
-
-// Sync returns the iterator to the position of another. It is assumed that the
-// iterators both operate on the same set of data.
-func (i *Iterator[T]) Sync(o *Iterator[T]) {
-	i.index = o.index
-}
-
 // Get returns the datum at the current position of the iterator.
 func (i Iterator[T]) Get() (T, error) {
 	var null T
@@ -93,14 +81,14 @@ func (i *Iterator[T]) While(fn func(T) bool) {
 }
 
 // Try attempts to perform an operation using the iterator. If the operation
-// succeeds, the iterator is updated. If the operation fails, the iterator is
-// rolled back, and an error is returned.
+// succeeds, the iterator keeps its new position. If the operation fails, the
+// iterator is rolled back, and an error is returned.
 func Try[T any, U any](i *Iterator[T], fn func(i *Iterator[T]) (U, error)) (U, error) {
-	i2 := i.Copy()
+	saved := i.index
 
-	out, err := fn(i2)
-	if err == nil {
-		i.Sync(i2)
+	out, err := fn(i)
+	if err != nil {
+		i.index = saved
 	}
 
 	return out, err