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>