Clamp a container child's line range to its parent

[Joncallim]

Summary

P3 from review pass 6. swift-markdown reports an unterminated fence inside a list item or block quote as running on through the following item/paragraph, so a child block's line range extended past its parent's (the reviewer measured 11,677 containment violations across 30,000 fuzzed documents). Export then treated the next item's $x$ as sitting inside that code block and silently did not typeset it (- ```js⏎ code⏎- item $x$). resolvedRanges now clamps a child's end to its parent's last line for nested levels; document level is deliberately left alone (clamping there surfaced extra top-level overlaps in the reviewer's testing).

Tests

The two reported shapes, plus 400 generated documents from structural fragments asserting that no child range escapes its parent (the reported shapes fail on master); MarkdownEngine, Export, Outline and Preview suites pass.

🤖 Generated with Claude Code

🤖 Generated with Claude Code