Split emphasis markers at paragraph breaks, merge heading lines

[xijo]

Summary

Two related fixes for markdown rendering issues when line breaks occur inside formatting tags:

Emphasis (strong/em) with paragraph breaks

When <br><br> occurs inside <strong> or <em>, markers must be split so they don't span across paragraph breaks.

Before: <strong>hello<br><br>world</strong> → **hello  \n  \nworld** (broken rendering)
After:  <strong>hello<br><br>world</strong> → **hello**  \n  \n**world** (correct)

Headings with line breaks

Markdown headings can't span multiple lines, so content is merged into a single line.

Before: <h1>foo<br>bar</h1> → # foo  \nbar (bar loses heading)
After:  <h1>foo<br>bar</h1> → # foo bar (all content in heading)

Implementation

• Added wrap_with_markers helper in Base class for splitting emphasis at paragraph breaks
• Updated H converter to merge lines with spaces

Test plan

• Added specs for strong, em, and h converters
• All 212 tests pass
• Verified roundtrip with Redcarpet renders correctly

Fixes #95

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