Initial design, plan, scaffold, and the full incremental implementation
of InstantPage V2 used by ChatMessageRichDataBubbleContentNode: layout
data types and driver, shared text helpers, text view / divider /
anchor, lists, code blocks, block/pull quotes, media placeholders,
details (with recursion), tables, hit-test / selection / last-line
helpers, V1→V2 swap in the rich-data bubble, and post-swap fix-ups
(cache key, RTL paragraphs, list geometry, lazy pageView construction,
fitToWidth horizontal inset, V2View.update() no longer writes
self.frame).
