Function parse_version_heading 

fn parse_version_heading(line: &str) -> Option<String>

Parses an ##-level Markdown heading and returns the version string, or None if the heading does not look like a version entry.

Recognised formats:

• ## [1.2.0] - 2024-01-15 (Keep a Changelog bracketed)
• ## [1.2.0] (bracketed, no date)
• ## 1.2.0 - 2024-01-15 (unbracketed with date)
• ## 1.2.0 (unbracketed, no date)