Consolidate button style resolution into StyleAttributeMapper

[chubes4]

Refs #242

Consolidate button style resolution into StyleAttributeMapper; behavior-preserving.

Why

ButtonStyleResolver (#241) independently re-parsed the same CSS color / typography / spacing / border declarations that the shared StyleAttributeMapper (#261) already resolves for every block. The overlap was literal, not coincidental:

• parseBorderShorthand() was byte-for-byte identical between the two classes.
• border(), backgroundColor(), boxSides()/padding(), and cssColor()/color() were near-duplicate CSS-parsing mechanics.

The only genuinely button-specific semantic (filled-vs-outline / is-style-outline) already lives in ButtonsPattern::hasOutlineSignal, not in ButtonStyleResolver. So ButtonStyleResolver was almost pure duplicated parsing plus a thin button presentation policy.

What

ButtonStyleResolver::nativeAttributes() now delegates the generic CSS→attribute parsing to StyleAttributeMapper::map() and keeps only the button presentation policy:

• color shape {background, text} (background before text), never a gradient fill;
• spacing.padding only (buttons don't carry margin — inter-button spacing rides on the parent core/buttons gap);
• the curated typography subset (fontSize, fontWeight, lineHeight, textTransform).

The string→declarations parser stays (the mapper takes a declaration array; the button entry point takes the resolved CSS string). ButtonsPattern and StyleAttributeMapper are untouched.

Net: -178 / +50 lines in ButtonStyleResolver.php; the duplicated CSS-parsing helpers are gone.

Behavior-preserving

The emitted style object — and its key order, which the parity runner compares with strict !== — is byte-identical. Baseline and post-change:

• composer parity: 144 fixtures pass (unchanged), filled → bg+text, ghost → is-style-outline + border+text, unstyled → default.
• Full composer test green (canonical + parity + packaging); php -l clean.
• No fixture edited.

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