Files above the editable size cap (MAX_VIEW_CHARS) now render their full
content through pierre's Virtualizer (viewport-only DOM) with highlighting on
the shared Shiki worker pool instead of a 200k-char main-thread-highlighted
slice.
Also fixes a pierre virtualization deadlock: forcing the virtualized host to
viewport height decoupled the IntersectionObserver judgment box from the
scroll content height, so a large scroll jump blanked the file (0 lines).
The large-file host now keeps no fixed height; the observer always sees it
intersecting.
Addresses #2868 (part 1: large-file preview virtualization).
gaojunran