docs(core): document PageScrollLockController like other 2nd-gen controllers - #6534

Description

Moves PageScrollLockController into the standard 2nd-gen controller folder layout (controllers/page-scroll-lock-controller/{index.ts,src/,stories/,test/,page-scroll-lock-controller.mdx}), matching color-controller, hover-controller, pending-controller, etc. No behavior change to the controller.

Adds:

• A Storybook docs page with usage, behavior, and accessibility guidance
• One demo host with a native lock/unlock toggle button
• A functional test covering reference-counted stacking, idempotency, and disconnect cleanup
• A ./controllers/page-scroll-lock-controller.js package export

Motivation and context

This was the only 2nd-gen controller without a folder, stories, or docs page.

Related issue(s)

• No tracked issue; documentation/organization cleanup.

Screenshots (if appropriate)

N/A — docs-only change.

Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed at the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

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

• Docs page renders

  1. Go to Storybook, Controllers/Page scroll lock controller
  2. Confirm Overview, Behaviors, and Accessibility sections render with no console errors

• Lock/unlock toggle

  1. Click "Lock page scroll" in the Overview demo — canvas becomes unscrollable
  2. Click "Unlock page scroll" — scroll is restored

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. Demo toggle is a native <button>; activates with Enter/Space
  2. Controller itself is unchanged, so existing consumers (e.g. swc-popover) see no keyboard regressions

• Screen reader (required — document steps below)

  1. Controller sets no ARIA and manages no focus; it only toggles overflow on document.documentElement
  2. No announcement changes for existing consumers