Open rendered code, diff and diagram blocks by clicking or arrowing in

Clicking a preview opens its source with the caret where the click was
(the clicked line and column for code, the matching patch line for a diff,
the end for a diagram or formula). Code and diffs swap the coloured copy for
the source instead of showing both. Escape returns to the preview and holds
focus on it; Enter, Space or typing on a focused preview opens the source,
and ArrowUp/ArrowDown move on. Arrowing from a neighbouring block enters a
collapsed block instead of skipping it: the caret goes into code, and a
diagram is held as a block. A source the caret opened closes again when the
caret leaves; one opened with the toggle stays open.

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