x-compact-markdown-file

When asked to "compact" a markdown file, perform a whitespace-normalization pass:

• Collapse multiple empty lines anywhere into a single empty line.
• For lists (bullets or numbered):
  • Remove empty blank lines between list items.
  • Remove empty blank lines at the end of a list (no trailing empty newline blocks after the final list item).

Do NOT change semantic content (no rewrapping paragraphs, no reordering list items, no changing indentation levels beyond what's required to remove empty lines).