Quiet rendered block chrome and steady diagram previews

Code, diff, diagram and formula blocks now share one corner control
pattern: no header bar at rest, a named fence's language as quiet text,
and the menu and source toggle revealed on hover or focus (always shown
in their own band on touch). Diagram blocks are labelled "Diagram"
everywhere, and empty code, diff and diagram blocks say what belongs in
them.

Diagrams a little wider than the measure scale to fit; wider ones keep
their size and scroll with a fade on the hidden side. A parse error no
longer replaces the drawing on each keystroke: the last good drawing
stays, and a quiet error appears only after 600 ms without a fix.

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