FE-1319: add button group to ds

[alex-e-leon]

🌟 What is the purpose of this PR?

Add ButtonGroup component

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• does not modify any publishable blocks or libraries, or modifications do not need publishing

📜 Does this require a change to the docs?

The changes in this PR:

• are internal and do not require a docs change

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
hash	Ready	Preview	Aug 5, 2026 4:25pm
hashdotdesign-tokens	Ready	Preview	Aug 5, 2026 4:25pm
petrinaut	Ready	Preview	Aug 5, 2026 4:25pm

[cursor]

PR Summary

Low Risk
New presentational DS component and a non-breaking data-variant attribute on Button; no auth, data, or app logic changes.

Overview
Adds a ButtonGroup layout component to @hashintel/ds-components for grouping buttons with spaced (gap) or segmented (shared borders) layouts, plus reverse, alignedTo, and noWrap.

Segmented styling uses a CVA recipe that squares interior corners, overlaps borders, and handles solid button gaps and tooltip wrapper nesting via data-variant and data-row-*. useSegmentedRows measures wrap breaks (ResizeObserver/MutationObserver) and clones children with row-start/end markers so corner radii stay correct on multi-line groups.

Button now sets data-variant (default solid) so the group can target child styles. The package exports ButtonGroup; Ladle stories cover wrapping, alignment, and mixed variants. Button stories get minor layout tweaks only.

^{Reviewed by Cursor Bugbot for commit 876552b. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 75a0faf. Configure here.}