Reach Diagram by Mermaid, keep long titles readable, hold error room

The language menu's type-to-jump also checks aliases, so "m" reaches
Diagram as well as "d". A long fence title now ellipsises at its end
instead of being clipped from its start on narrow screens. A diagram
whose source is open reserves the error line's height, so a parse error
arriving after a pause no longer pushes the source down.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>