Migrate off the deprecated gradient shader API

[LeadcodeDev]

Bumping to skia-safe 0.99 (#214) leaves 25 deprecation warnings, all on the
gradient shader API. They are silenced with narrowly scoped #[allow(deprecated)]
in that PR so the docs.rs fix can land; this issue is the follow-up.

Why it was not done in #214

It is a restructuring, not a rename. The old call takes the pieces flat:

gradient_shader::linear(
    (p0, p1),
    colors.as_slice(),
    positions.as_slice(),
    TileMode::Clamp,
    None,
    None,
)

The new one takes a Gradient struct and drops TileMode from the signature
entirely:

gradient::shaders::linear_gradient(
    points: (impl Into<Point>, impl Into<Point>),
    gradient: &Gradient<'_>,
    local_matrix: impl Into<Option<&Matrix>>,
) -> Option<Shader>

Gradient::new(colors: Colors, interpolation: impl Into<Interpolation>). Where the
tile mode now lives has to be established before touching anything — getting it
wrong changes rendering silently, and a gradient that clamps where it used to
repeat looks plausible in isolation.

Scope

25 sites across 8 files:

File	Sites
rustmotion-core/src/engine/paint_pass.rs	6
rustmotion/src/engine/render/background.rs	5
rustmotion-components/src/shape.rs	4
rustmotion-components/src/stat.rs	2
rustmotion-components/src/sparkline.rs	2
rustmotion-components/src/skeleton.rs	2
rustmotion-components/src/gradient_text.rs	2
rustmotion-components/src/chart/line.rs	2

Both GradientShaderColors::Colors and ::ColorsInSpace are in use, so the
colour-space carrying case has to survive too.

How to verify

The pixel regression suite is the judge. A gradient that renders identically
before and after is the only acceptable outcome; anything else means the tile
mode or the interpolation moved.

Removing the #[allow(deprecated)] attributes is part of the fix — leaving one
behind would hide the next deprecation in the same function.