Initialize standard JSON pair_pos sentinel

[SRO-SA]

Fixes #3

Summary:

• Zero-initializes the pinned host result buffer in parse_standard_json.
• Explicitly initializes the artificial root/sentinel slot shared by structural[result_size + 1] and pair_pos[0].
• Preserves the existing parser layout where real structural data starts at structural[1] and real pair_pos data starts at pair_pos[1].

Testing:

• I could not run CUDA or compute-sanitizer locally because my laptop does not have CUDA access.
• I reviewed the standard JSON result-buffer layout manually.
• Please rerun the initcheck reproducer from the issue to confirm the uninitialized read is resolved.

Note:

• I kept the existing +2 pair_pos copy offset because the code reserve pair_pos[0] for the artificial root/sentinel entry. Moving the copy destination to +1 would shift real pair_pos entries and may break iterator semantics.