chore(close-button): add structure for s2 migration

[TarunAdobe]

Description

Phase 2 (Setup) of the swc-close-button 1st-gen → 2nd-gen washing machine migration. This PR scaffolds the component so it builds, registers, and appears in Storybook before API polish and S2 styling land in follow-up PRs.

Core (2nd-gen/packages/core/components/close-button/)

• CloseButton.types.ts — CLOSE_BUTTON_VALID_SIZES, static-color / variant types
• CloseButton.base.ts — extends ButtonBase with static-color, legacy variant (maps to static-color when set; full deprecation warnings deferred to Phase 3), and 1st-gen noDefaultSize sizing behavior
• Package export wired in core/package.json
  SWC (2nd-gen/packages/swc/components/close-button/)
• CloseButton.ts — inner <button type="button">, size-mapped cross icon, visually hidden default slot, aria-label from resolved accessible name
• close-button.css — stub layout and visually hidden label styles (not S2 visual parity)
• swc-close-button.ts — element registration
• Storybook Playground + Overview stories
• Placeholder unit and a11y spec tests (expanded in Phase 6)

Docs

• Migration status table: Close Button Setup marked complete

Motivation and context

Close button migration is tracked under the 2nd-gen component workstream. Setup must land before Phase 3 (API / TypeScript) and Phase 5 (S2 CSS). This PR intentionally ships a minimal, buildable skeleton — not visual parity with Spectrum 2 closebutton CSS.

Related issue(s)

• Epic SWC-2087 — Close button migration
• Planning (separate PR): SWC-2089 — migration plan
  (#6322)

Screenshots (if appropriate)

N/A — stub styling only; no meaningful visual regression vs Spectrum 2
yet.

Author's checklist

• I have read the [CONTRIBUTING](https://github.com/adobe/spectru
  m-web-components/blob/main/CONTRIBUTING.md) and
  [PULL_REQUESTS](https://github.com/adobe/spectrum-web-components/blob
  /main/PULL_REQUESTS.md) documents.
• I have reviewed the Accessibility Practices for this feature,
  see: Aria Practices
• I have added automated tests to cover my changes (placeholder
  Storybook + a11y snapshot; full coverage in Phase 6).
• I have included a well-written changeset if my change needs to be
  published.
• I have included updated documentation if my change required it
  (status table only).

---

Reviewer's checklist

• Includes a Github Issue with appropriate flag or Jira ticket
  number without a link
• Includes thoughtfully written changeset if changes suggested
  include patch, minor, or major features
• Automated tests cover all use cases and follow best practices for
  writing
• Validated on all supported browsers
• All VRTs are approved before the author can update Golden Hash

Manual review test cases

• Verify close-button scaffold builds and registers
  1. Run yarn build from repo root (or 2nd-gen workspace build for
     core + swc).
  2. Confirm build completes without errors.
• Verify element registration and Storybook
  1. Open 2nd-gen Storybook → Close Button → Playground (or
     branch preview Storybook if available).
  2. Confirm <swc-close-button accessible-label="Close"> renders with
     a cross icon.
  3. In DevTools, confirm shadow DOM contains a real inner <button type="button"> with aria-label="Close".
• Verify size and static-color attributes (stub visuals)
  1. In Playground, set size to s, m, l, and xl; confirm the
     host updates and the icon mapping changes (l/xl may share the largest
     available 2nd-gen cross icon until Cross400/500 land).
  2. Set static-color to white or black; confirm classes
     swc-CloseButton--staticWhite / swc-CloseButton--staticBlack appear
     on the inner button (styling is stub-only).
• Verify no reflected size when unset
  1. Render <swc-close-button accessible-label="Close"> with no
     size attribute.
  2. Inspect the host; expect no size attribute on the host
     (visual default still behaves as m).

Device review

• Did it pass in Desktop?
• Did it pass in (emulated) Mobile?
• Did it pass in (emulated) iPad?

Accessibility testing checklist

• Keyboard (required — document steps below)
  1. Open 2nd-gen Storybook → Close Button → Playground.
  2. Press Tab until focus reaches the close button; confirm
     a visible focus indicator on the inner button (delegated focus).
  3. Press Enter or Space; confirm the button
     activates (click handler / default button behavior).
  4. Set disabled on the control; confirm the button is not reachable
     via Tab and does not activate.
• Screen reader (required — document steps below)
  1. Open Playground with VoiceOver (macOS) or NVDA (Windows).
  2. Tab to the button; confirm it is announced as a button named
     Close (from accessible-label or slotted text).
  3. With disabled set, confirm the control is announced as
     unavailable/disabled.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 7e643ab

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[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-6355

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.