refactor(mix): deprecate the per-edge border helpers

[leoafarias]

BorderStyleMixin exposed nine helpers — borderAll, borderTop, borderBottom, borderLeft, borderRight, borderStart, borderEnd, borderVertical, borderHorizontal — each taking {color, width, style, strokeAlign}. Every one composes exactly what border() already expresses through BoxBorderMix, so they duplicated the property surface without adding capability.

// Before
BoxStyler().borderAll(color: Colors.blue, width: 1.5);
BoxStyler().borderTop(color: Colors.blue, width: 2);

// After
BoxStyler().border(.color(Colors.blue).width(1.5));
BoxStyler().border(.top(.color(Colors.blue).width(2)));

The dot-shorthand form also reads better: BoxBorderMix exposes the edges as statics and BorderSideMix chains color/width/style/strokeAlign, so nothing names a type.

BoxBorderMix.all() is deliberately not deprecated

It isn't redundant, and it's worth separating from the above:

• It's the only way to apply a prebuilt BorderSideMix to every edge — .color().width() can't express that.
• It backs BoxBorderMix.none.
• It stays symmetric with .top() / .bottom() / .left() / .right(), which all take a BorderSideMix.

Verification

• All nine replacements were compiled before the deprecation messages were written. The first draft suggested .borderDirectional(...) for start/end — that doesn't exist. start and end are statics on BoxBorderMix, so .border(.start(...)) is correct. A deprecation pointing at a non-existent API would be worse than none.
• Zero call sites anywhere in the repository, including tests, so nothing internal changes.
• flutter test — 2918 passing. flutter analyze — clean. dart format — clean.

Not included

SpacingStyleMixin.paddingAll(double) is literally padding(EdgeInsetsGeometryMix.all(value)) — the identical shape. But it has 29 usages across the repo, so it's the same argument with a much larger blast radius and belongs in its own decision.