Preserve fragment form pointer across seeks

PR #61: Preserve fragment form pointer across seeks

PR #61 updates the fragment parser to preserve the form pointer across backward seeks. This change prevents the loss of the form pointer when the fragment parser rewinds using the seek() method, ensuring correct parsing of nested forms. By storing the fragment context’s form pointer and restoring it during backward seeks, the parser maintains the correct state for accurate handling of nested forms.

This PR also removes an obsolete TODO comment related to form pointer invalidation and adds a regression test to verify that nested forms are ignored before and after a backward seek. This ensures consistent behavior and reliability of the parser in complex scenarios, reducing potential parsing errors from improper state management during backward navigation.