The parse cache hands back shared mutable file objects, and a FileDiff
instance keys its render and highlight bookkeeping on them - the same
reason DiffPanel scopes its cache per theme. With the simple and
advanced typography views both consuming one cached parse, a remounted
preview could inherit stale already-highlighted state and render plain
text. Each mount now parses under its own scope.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>