feat(button-group): 2nd-gen component with API, a11y, storybook, and tests

[Rajdeepc]

Summary

Implements the full 2nd-gen <swc-button-group> component covering file structure, API, accessibility, Storybook documentation, and test suites.

What's included

• Core (ButtonGroupBase): Abstract base class with orientation, align, disabled, size propagation to slotted children, role="group" semantics, and dev-mode validation warnings
• SWC (ButtonGroup): Concrete class with S2-token-based CSS, classMap rendering for all variants
• Storybook: Full documentation with getStorybookHelpers args table, Figma link, Stackblitz link, upcoming features section, accessibility guidance, and production vs local-only story designation
• Tests: Storybook play-function unit tests (5 standard sections: defaults, properties, slots, variants, dev warnings) and Playwright a11y tests (ARIA snapshots, keyboard interactions, aXe validation)
• SizedMixin: Marks _size field as @internal

Breaking changes from 1st-gen

Change	1st-gen	2nd-gen
Tag name	sp-button-group	swc-button-group
Orientation	vertical boolean	orientation="horizontal|vertical"
CSS custom properties	--mod-buttongroup-*	--swc-button-group-*
Default size	None (noDefaultSize)	m
ARIA	No role set	role="group" automatic

New APIs

• align: start | center | end — main-axis alignment
• disabled: Group-level disable propagated to children

Test plan

• yarn build passes in both core and swc packages

• ESLint and stylelint pass with no errors

• Storybook renders all stories without console errors

• Play-function tests pass via test:storybook

• Playwright a11y tests pass via test:a11y

• aXe reports no WCAG violations

• Visual review in Storybook for all sizes, orientations, alignments

• Fixes SWC 2074, SWC-2075, SWC-2076

[changeset-bot]

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Latest commit: 7109c6d

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[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6334

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.