Line breaks inside multi-line HTML comments are collapsed

## Summary

When an HTML comment spans multiple lines, flowmark collapses the internal line breaks, joining all content onto a single line. Flowmark already preserves line breaks *around* HTML comment tags (before/after ``), but does not preserve line breaks *within* them.

## Version

flowmark 0.6.4

## Reproduction

Run `flowmark --auto` on a markdown file containing multi-line HTML comments:

```markdown

```

## Observed behavior

All internal line breaks are collapsed to a single line:

```markdown

```

This produces lines that can exceed 300+ characters for comments with many attributes.

## Expected behavior

Line breaks within HTML comments should be preserved as-is. The internal formatting of comments is intentional and should not be reflowed.

## Use case

[Markform](https://github.com/jlevy/markform) uses HTML comments with structured attributes for form field definitions. These are intentionally formatted across multiple lines for readability, with attributes like `columnIds`, `columnLabels`, and `columnTypes` on separate lines. Collapsing them makes them very hard to read and review in diffs.

This likely also affects any other use of multi-line HTML comments in markdown (e.g., template directives, metadata annotations, etc.).