test(classic): add release-readiness coverage

[dancormier]

Summary

• validate the packed Stacks Classic tarball, metadata, declarations, CSS, Less, and both UMD bundles through a clean consumer
• run the package smoke test in Chromium as a required CI check
• document the supported V3 package entry points and major-version migration paths
• correct the repository release documentation to match the current main-based Changesets workflow

Jira

• STACKS-868

Validation

• npm run test:package -w packages/stacks-classic
• npm run lint -w packages/stacks-classic
• npm run test:less -w packages/stacks-classic — 40 passed
• npm run test:unit -w packages/stacks-classic -- --config web-test-runner.config.ci.mjs — 96 passed
• npm run test:a11y -w packages/stacks-classic -- --config web-test-runner.config.ci.mjs — 2,589 passed; 155 existing skips
• npm run test:visual:ci -w packages/stacks-classic -- --config ./visual-runner/stacks-classic-runner-config/web-test-runner.config.ci.mjs — 8,508 passed
• git diff --check

Release notes

• no changeset: this PR adds tests and documentation without changing the published package behavior
• versioning, prerelease exit, dependency promotion, and publication remain part of the coordinated stable release

Accessibility

• no UI behavior changed
• the existing Classic accessibility suite passes with 2,589 checks and 155 pre-existing skips

[changeset-bot]

⚠️ No Changeset found

Latest commit: df72e7f

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

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

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[dancormier]

Note

This PR is part of a larger effort to close out the beta phase of this repository.

See also: #2365