fix(paint): let a self-painting component see its animated background

[LeadcodeDev]

Closes #324.

Reproduced

A badge with a timeline step changing style.background, over a 1.0 s linear
transition:

	before	after
t = 0.2 s	#FF0000	#FF0000
t = 1.0 s	#FF0000	#7F007F
t = 1.8 s	#FF0000	#0000FF

Cause

Six components paint their own background and read it from their own style:
badge, callout, caption, kbd, stat, tooltip.

resolve_transition_css_overrides writes the interpolated colour onto the node's
css.background. The cascade only carries inheritable properties onto the
clone a component is painted from, and background is not one — so the component
kept painting its declared colour, over the animated one the generic background
step had just painted underneath it.

The fix

with_resolved_background writes the node's resolved background onto a clone of
those six. It returns None when the resolved value already equals the
component's own, so a component whose background is not animated is not cloned
at all — the serde round-trip is not paid per frame for the common case.

Why not fold it into with_cascaded_style

stat. That function is about inheritance and classifies stat as
non-typographic, so it returns None for it — folding the fix in there would have
missed a sixth of the components it is for. The two concerns are kept apart, and
a_stat_follows_it_too_although_the_cascade_never_clones_it is the test that pins
that specific path.

Tests

Test
a_badge_follows_an_animated_background_instead_of_repainting_its_own	the typographic path
a_stat_follows_it_too_although_the_cascade_never_clones_it	the path with_cascaded_style does not reach
a_component_with_no_background_animation_is_untouched	the clone is not taken when there is nothing to carry

Reverting the dispatch change fails the first two with got (255, 0, 0) — the
declared colour, at the halfway point.

Gate

cargo fmt --all --check clean · cargo clippy --workspace --all-targets --features rustmotion/studio -D warnings clean · cargo test --workspace 1787 passed.