perf(json): keep array scan linear on malformed output

[Yif-Yang]

Summary

Immediate maintainer follow-up to #103, which was merged first with the contributor commits and attribution intact.

The unmatched-brace recovery added by #103 is functionally correct, but it looked ahead to EOF for every unmatched {, making malformed brace-heavy model output quadratic. This change:

• precomputes balanced object endpoints in one string/escape-aware stack pass;
• preserves the behavior where an unmatched prose brace does not hide a later valid array;
• continues to skip arrays nested inside balanced objects; and
• adds a deterministic character-access regression test rather than a timing-based assertion.

Validation

• pytest tests/test_json_utils.py -q: 40 passed, 3 skipped
• pytest tests/ -q: 276 passed, 6 skipped
• 10,000 unmatched braces: approximately 4.8 s → 0.002 s
• 100,000 unmatched braces: approximately 0.022 s
• git diff --check: clean
• Claude Code / Opus 4.8 read-only review: no blockers, MERGE