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/token/parse.go

@@ -8,17 +8,18 @@ import (
 
 // ParseRawToken consumes the next token from the iterator if its type matches
 // the expected type.
-// Uses [iterator.Try] for automatic backtracking on failure.
+// Returns an error if the iterator is exhausted or the token type does not
+// match.
 func ParseRawToken[T Type](i *iterator.Iterator[Token[T]], expected T) (*Token[T], error) {
-	return iterator.Try(i, func(i *iterator.Iterator[Token[T]]) (*Token[T], error) {
-		if tok, err := i.Next(); err != nil {
-			return nil, err
-		} else if tok.Type != expected {
-			return nil, fmt.Errorf("expected token %v, got %v'", expected.Name(), tok.Value)
-		} else {
-			return &tok, nil
-		}
-	})
+	tok, err := i.Get()
+	if err != nil {
+		return nil, err
+	}
+	if tok.Type != expected {
+		return nil, fmt.Errorf("expected token %v, got %v'", expected.Name(), tok.Value)
+	}
+	i.Forward()
+	return &tok, nil
 }
 
 // ParseList repeatedly applies a parse function, collecting results into a

pkg/token/scan.go

@@ -17,15 +17,15 @@ func IsVariable(r rune) bool {
 // predicate.
 // Returns an error if the iterator is exhausted or the rune does not match.
 func ScanRune(i *iterator.Iterator[rune], expected func(rune) bool) (rune, error) {
-	return iterator.Try(i, func(i *iterator.Iterator[rune]) (rune, error) {
-		if r, err := i.Next(); err != nil {
-			return r, err
-		} else if !expected(r) {
-			return r, fmt.Errorf("got unexpected rune %v'", r)
-		} else {
-			return r, nil
-		}
-	})
+	r, err := i.Get()
+	if err != nil {
+		return r, err
+	}
+	if !expected(r) {
+		return r, fmt.Errorf("got unexpected rune %v'", r)
+	}
+	i.Forward()
+	return r, nil
 }
 
 // ScanCharacter consumes the next rune from the iterator if it matches the