fix line misalignment when code block is between headings

[jackhogan]

When rm_fenced=True, line indices from text_for_headings.splitlines() (after fenced block removal) were being used to slice original_lines (before removal), causing misalignment.

For example:

md_content = """# First Header
Content here.

```python
# A comment that should be ignored for structure
def example():
    pass
```

# Second Header
This content belongs to Second Header."""

create_heading_dict(md_content)['Second Header'].text

results in

'def example():\n    pass\n```\n\n# Second Header\nThis content belongs to Second Header.'

This wasn't being caught in the tests because there was no example of a fenced code block between two headings.