The user message bubble had hover on a `Pressable` wrapping the bubble,
with the timestamp and copy button as siblings outside it — moving the
cursor onto them lost hover. Wrapping both in a single Pressable then
flickered, because `TurnCopyButton` is itself a Pressable and nested
Pressables fight over hover state.
Rewrite `UserMessage` to match the workspace row pattern: hover-tracking
on a plain outer `View` with `onPointerEnter`/`onPointerLeave`, nested
Pressables freely inside. Pointer events are non-bubbling DOM events,
so descendants can't disrupt the outer hover.
Rewrite `docs/hover.md` with the workspace row as THE canonical pattern,
not one option among several. Names the three failure modes that come
from skipping it — including "nested Pressables fight over hover" — and
explicitly bounds when `Pressable.onHoverIn`/`onHoverOut` is acceptable
(self-styling only).
Update `constants/platform.ts` so its hover guidance points at the doc
instead of recommending the broken pattern.
