fix(skia): migrate to skia-safe 0.99 so docs.rs can build

[LeadcodeDev]

rustmotion 0.6.1 published with no documentation: docs.rs could not build it.
This moves the workspace to skia-safe 0.99, where that build works.

Closes #214. Follow-up in #215.

The cause

skia-bindings 0.82.0 has a docs.rs-specific path in its build script, and it is
broken:

DETECTED DOCS_RS BUILD
COPYING bindings_docs.rs to OUT_DIR/skia/bindings.rs
Error: Os { code: 2, kind: NotFound, message: "No such file or directory" }

Not ours: skia-safe 0.82.0 has no docs on docs.rs either. 0.99.0 does.

What actually broke

One thing. Skia made SkPath immutable and moved construction into
SkPathBuilder, so the mutating methods left Path.

let mut path = PathBuilder::new();
path.move_to(a);
path.line_to(b);
path.close();
canvas.draw_path(&path.detach(), &paint);

22 of the 99 files that touch skia_safe needed changing. Canvas, Paint,
Surface, ImageInfo, FontStyle, Shader and Data crossed seventeen minor
versions untouched.

Three details a reviewer should not have to rediscover:

• add_rect, add_oval and add_rrect gained a third argument (start_index).
  add_circle already had three and is unchanged.
• Path::from_svg still returns a real Path. A blanket rewrite of
  draw_path(&path puts a .detach() there. It does not compile, which is
  fortunate — the failure mode had it compiled would have been silent.
• chart/radar.rs draws one path twice, filled then stroked. It is the only site
  where snapshot() is right and detach() would empty the builder too early.

Path::get_point is deprecated; replaced by points().first(), same semantics.

The gradient API, migrated too

25 sites were initially held under #[allow(deprecated)] and the migration
deferred to #215. That is no longer the case — there is no legacy left on this
branch, and #215 is closed by this PR.

Why those functions were deprecated at all (skia-safe 0.93): Skia gained CSS
Color 4 style interpolation, and the old flags: u32 could only carry one
boolean. The new model needs three orthogonal parameters — premultiplied or not,
which colour space to interpolate in, how to traverse hue — so the API moved to
Gradient { Colors, Interpolation }.

Rendering is preserved by construction, not by hope:

• the old Flags::default() was empty(), the new Interpolation::default() is
  in_premul: No, and no site in this repository ever passed flags
  explicitly — so the defaults line up;
• every call site uses Interpolation::default(); no colour space is introduced
  where one was not already present. shape.rs keeps the srgb space it declared;
• TileMode did not disappear from the API, it moved into Colors next to the
  colours it applies to.

One implicit behaviour had to be made explicit: the deprecated sweep() took an
optional angle range where None meant a full 0..360 turn. The replacement makes
the tuple mandatory, so that range is now written at the call site.

Verification

• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test --workspace — 1241 passed, 0 failed, 10 ignored

The pixel regression suite is what matters here, not the compiler. A misplaced
detach() compiles perfectly and renders a missing shape; those tests are what
say the paths still draw the same.