Reveal raw --- (no rule) when cursor is on the HR line
The cm-hr line decoration that draws the rule was applied unconditionally,
so placing the cursor on a horizontal rule showed both the rule and the raw
"---". Only style the line (and hide its glyphs) when the cursor is on a
different line; on the rule's own line, emit nothing so the raw "---" shows
for editing — matching how headings/blockquotes reveal their markers.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>