feat(mix): accept transform in gradient shorthands

[leoafarias]

Summary

linearGradient, radialGradient, sweepGradient and the three foreground*Gradient shorthands on DecorationStyleMixin now take an optional GradientTransform? transform. It is passed straight to the matching LinearGradientMix, RadialGradientMix or SweepGradientMix. The generated BoxStyler, FlexBoxStyler, StackBoxStyler and WrapBoxStyler factories were regenerated and forward it too.

Before, spinning a gradient (for example a GradientRotation driven by a keyframe track) meant dropping to gradient(SweepGradientMix(...)), even though every gradient Mix type already supports transform.

style.sweepGradient(
  colors: [Colors.cyan, Colors.purple, Colors.cyan],
  transform: GradientRotation(angle),
)

Why extend the shorthand instead of deprecating it

• These shorthands are not deprecated and do not appear under "Retired Conveniences" in skills/mix/references/styler-api-policy.md. They are grouped, nullable-argument conveniences, which the policy keeps.
• They already forward stops and tileMode. transform was the only gradient field they did not expose, so this change makes them complete rather than adding a new kind of API.

Changes

• decoration_style_mixin.dart: adds transform to all six gradient shorthands, with doc notes.
• Regenerated *_spec.g.dart factories for Box, FlexBox, StackBox and WrapBox.
• Tests in box_style_factory_test.dart: each shorthand resolves with the given transform; the result equals the explicit gradient(...Mix(transform: ...)) form; static factories match the instance methods; omitting transform leaves it null.
• Docs: skills/mix/references/fluent-api.md table, and a "Spinning a gradient" keyframe example in animations.md. The example was checked with a temporary widget test that was not committed.
• CHANGELOG: new Unreleased entry.

Additive and non-breaking: existing calls are unchanged.

Validation

• fvm flutter test in packages/mix: 2986 tests passed
• fvm dart analyze packages/mix: no errors or warnings. The 7 prefer_initializing_formals infos are all in files this PR does not touch.
• dart format: clean