chore(button-group): 2nd-gen migration plan (#6333)

* feat(button-group): gen2 migration plan

- document 1st-gen API surface (vertical, size, slot, CSS tokens)
- define 2nd-gen API decisions aligned with React Spectrum S2 and Figma
- identify breaking change: vertical boolean replaced by orientation
property
- capture accessibility requirements from migration analysis (role=group,
aria-orientation, no roving tabindex)
- include Figma S2 Desktop reference screenshot
- flag open questions for XL size support and overflow behavior scope
- register migration plan in components README navigation

Epic: SWC-2071
Ref: SWC-2073
Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(button-group): remove figma screenshot from repo

- binary assets should not be committed to the repository

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(button-group): address 5t3ph review feedback on migration plan

- Remove aria-orientation: not valid on role="group" since button-group
  does not implement focus group / arrow-key navigation
- Resolve Q1: XL is supported by React S2 and uses same 12px gap token
  as M/L; mark as confirmed in type definition
- Update Q2: incorporate feedback that regular flex wrapping may be an
  acceptable starting point; mark as deferred from MVP

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Rajdeep Chandra <rajdeepc@adobe.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

Author: 3 people

CONTRIBUTOR-DOCS/03_project-planning/03_components/README.md

@@ -47,6 +47,7 @@
     - [Button migration roadmap](button/rendering-and-styling-migration-analysis.md)
 - Button Group
     - [Button group accessibility migration analysis](button-group/accessibility-migration-analysis.md)
+    - [Button group migration plan](button-group/migration-plan.md)
     - [Button Group migration roadmap](button-group/rendering-and-styling-migration-analysis.md)
 - Checkbox
     - [Checkbox migration roadmap](checkbox/rendering-and-styling-migration-analysis.md)