[Stage 1.6] Migrate and validate the v2 release workflow for the monorepo

[draedful]

Parent: #318

Goal

Prove that v2 can be released and consumed from npm after the repository becomes a pnpm monorepo.

The workflow is a production delivery path, not only a build check.

Scope

• Keep release-v2.yml triggered by the v2 branch before cutover.
• Replace root-package npm assumptions with the Stage 1 pnpm workspace and package build model.
• Build and publish only public packages.
• Prevent private workspace packages from being published.
• Prevent unresolved imports or published dependencies on private packages.
• Permit only 2.x prereleases from v2.
• Publish under next; never update latest before cutover.
• Keep release creation behind a reviewed release pull request or equivalent approval.
• Verify generated versions, changelogs, Git tags, GitHub Releases, package manifests, and npm artifacts.
• Install the published prerelease from npm in clean vanilla JavaScript, React, and applicable Playwright consumer fixtures.
• Document the cutover-only configuration change from v2/next to main/latest.
• Provide a dry-run for the future stable v2 release path.

Acceptance criteria

• release-v2.yml runs from v2 using pnpm and the monorepo package graph.
• Only intended public packages are publishable.
• A 2.x prerelease is published under next after explicit approval.
• The prerelease does not update latest.
• Version, changelog, tag, GitHub Release, manifest, and npm artifact agree.
• Published packages contain no unresolved private-package imports or dependencies.
• Clean consumer fixtures install from npm without workspace access.
• Root, React, and applicable Playwright entry points pass runtime and type checks.
• The main/latest cutover configuration is covered by a safe dry-run.