perf(core): let views that paint their own box styles skip derived box state - #11595

Lets a view tell core it paints its own padding, borders and backgrounds, so core can skip the state it would derive from them.

What changes

A view can set _appliesOwnBoxStyle = true. For those views:

• style values are stored, read back and cascaded exactly as before;
• core skips the derived state it builds from them:
  • effectivePadding* and effectiveBorder*Width;
  • the Background object it rebuilds on every background, border, radius, box-shadow or clip-path change.

Views that don't set the flag behave exactly as before.

The use case is a layout plugin that draws boxes itself. @triniwiz/nativescript-masonkit paints these properties natively and never reads the derived state, but core still rebuilt it for every declaration on every view. Older cores ignore the flag, so the plugin can set it unconditionally.

Numbers

xplat-benchmarks ns-core-mason-perf, with masonkit setting the flag, size M, interleaved runs. Time to attach:

Screenshots were identical.

Testing

• New own-box-style.spec.ts. A flagged view keeps its style values but skips the derived paddings, border widths and Background. An unflagged view still derives them.
• All core unit tests pass (645).