docs: clarify v2 beta installation guidance

[wadakatu]

Summary

• mark Gesso 2 installation commands as prerelease evaluation guidance
• use the explicit Composer ^2.0@beta stability opt-in across README, quickstarts, and example READMEs
• add a documentation policy test that follows release-please's configured beta/stable mode
• document that stable promotion and beta recovery must update the installation guidance in the same PR as the release mode
• rebase the change onto the current main after the beta.4 and documentation-site updates

Why

Gesso 2 is currently published as v2.0.0-beta.4. An unqualified install command can read as a stable recommendation even though downstream validation is still part of the beta process. The documentation should make the release status and adoption boundary explicit.

The policy test prevents the beta warning and stability flag from remaining during stable promotion. It reads release-please-config.json rather than the release manifest so the normal promotion PR can update the release mode and documentation together; otherwise the bot-managed stable release PR would be the first place the manifest becomes stable and would fail its own CI.

Validation

• clean Packagist resolution of studio-design/gesso:^2.0@beta selected v2.0.0-beta.4
• php84 vendor/bin/phpunit tests/Unit/Build/DocumentationInstallPolicyTest.php — 1 test, 38 assertions
• PHP-CS-Fixer, both configurations, sequential — clean
• PHPStan with single-process sandbox settings — no errors
• npm run docs:build
• npm run docs:verify-accessibility
• npm run docs:verify-base
• npm run docs:verify-tombo
• npm run docs:verify-version
• git diff --check

ローカル実機確認

• 環境: 生成済み docs/.vitepress/dist/quickstarts/core.html / Chrome DevTools MCP
• 操作: Core / PHPUnit quickstartを1440pxと500pxで表示し、beta警告、Composerコマンド、ページ幅を確認
• 結果: Pre-release evaluation only と composer require --dev "studio-design/gesso:^2.0@beta" が読み上げツリーと画面の両方に表示され、PCではコマンドにoverflowなし、モバイルではコードブロック内だけスクロールしページ全体の横overflowなし
• Before/After: 無警告・無指定インストールコマンドから、beta評価用途の警告と明示的な安定度opt-inへ変更
• Console/Network: ローカルHTTPサーバーを利用できないため file:// で確認。CSSは読み込み済み。module scriptとローカルfontはブラウザーの file:// CORS制約で未読込
• 証跡: /private/tmp/gesso-pr375-beta4-desktop.png、/private/tmp/gesso-pr375-beta4-mobile.png
• 未確認事項: HTTP配信時のクライアントサイドナビゲーション。静的な表示内容とレスポンシブレイアウトは生成HTML/CSSで確認済み