fix: Preserve Markdown inline boundary spaces

[EmilStenstrom]

Summary

Preserve collapsed whitespace that appears just inside a, b, strong, em, and i elements when serializing to Markdown. This reimplements the behavior reported in #78 against the current Markdown builder.

The fix records leading and trailing collapsed whitespace in child builders and feeds it back through the parent builder's ordinary whitespace handling. It also avoids duplicate spaces after heading and list markers.

Inline child builders inherit the active blockquote depth, so formatting and link elements cannot reset the existing bound on deeply nested blockquote prefixes.

User impact

Rich-text HTML such as <p>This is <b>very </b>important.</p> now serializes as This is **very** important. instead of joining the surrounding words. Equivalent link and emphasis boundaries behave consistently.

Validation

• pre-commit run --all-files
• 100% test coverage
• parser differential at 100%
• focused Markdown and DOM test suite