gpui: Reconcile hover listeners after layout changes (#62335)

GPUI's declarative hover styles are recomputed from the current frame's
hit test, but `on_hover` listeners previously updated only in response
to mouse movement or the pointer leaving the window. When layout moved a
different element beneath a stationary pointer, the new element looked
hovered without receiving its hover-start callback.

This change reconciles each hover listener against the current hit test
during painting and defers transitions until after the paint cycle, when
application callbacks can safely update state. Reconciliation pauses
while a mouse press is pending so hover-only controls remain mounted
between mouse-down and mouse-up.

The regression coverage moves an element beneath and away from a
stationary pointer and verifies both transitions. It also verifies that
repainting during a stationary mouse press does not generate a spurious
hover exit.

Testing:

- `cargo test -p gpui --lib`
- `cargo fmt --check -p gpui`
- `cargo check -p delta --bin delta` with Delta temporarily patched to
the local GPUI checkout

Release Notes:

- Fixed hover interactions not updating when interface elements move
beneath a stationary pointer.