fix(seo): define a bounded canonical URL strategy for NPM Stats

src/routes/stats/npm/index.tsx から始め、特にルートのバリデーションと staticData.includeSearchInCanonical を確認してから、canonical、構造化データ、内部リンク、sitemap の挙動を追跡します。実装前に、意味論と表示を分離した範囲限定の URL モデルを定義し、デフォルト、パッケージ、スコープ付きパッケージ、比較、UI のみ、無効な組み合わせについてテストを追加します。完了の定義は、安定した意味のある URL だけがインデックス可能で、そのメタデータが一致することです。

Problem

The clean /stats/npm URL currently redirects to a URL that serializes the complete default chart state. In a live audit, the resulting canonical URL was 2,355 characters long.

The route validates defaults for packageGroups, range, chart type, transforms, playback controls, legend state, dimensions, and other UI settings, then staticData.includeSearchInCanonical causes all of those values to become canonical. Each user interaction can therefore create another indexable URL.

Relevant route: src/routes/stats/npm/index.tsx

Google warns that parameter-driven faceted navigation can generate effectively infinite crawl spaces and slow discovery of useful pages: https://developers.google.com/crawling/docs/faceted-navigation

Goals

• Keep NPM Stats URLs shareable.
• Index only clean, stable pages with meaningful search intent.
• Prevent display and playback controls from creating indexable canonical variants.

Proposed direction

• Make /stats/npm a stable 200 URL with a clean self-canonical, or normalize to another intentionally short canonical URL.
• Omit default values from serialized search parameters.
• Separate semantic state from presentation state.
• Treat parameters such as range, transform, bin type, view mode, chart type, sorting, dimensions, playback, legend visibility, and similar controls as non-indexable UI state.
• Define a bounded format for package or package-comparison pages that should be indexable, potentially using stable path-based package identifiers rather than encoded JSON.
• Canonicalize or noindex non-semantic variants according to the chosen model.
• Ensure scoped package paths resolve correctly and have one canonical encoding.
• Make WebApplication JSON-LD url and any BreadcrumbList URLs match the selected canonical.
• Review FAQPage JSON-LD. Google does not normally show FAQ rich results for non-government and non-health sites, so retain it only if another consumer justifies the maintenance.

Acceptance criteria

• Loading /stats/npm does not redirect to a URL containing the full default chart configuration.
• The default canonical is short, stable, and self-referencing.
• Default values are omitted from generated links.
• Presentation-only variants do not self-canonicalize as independently indexable pages.
• Any intentionally indexable comparison page has unique SSR title, description, H1, canonical, and structured data based on the compared packages.
• Nonsensical or empty package combinations return an appropriate 404 or noindex response rather than another indexable state.
• Tests cover default state, one package, scoped packages, multi-package comparison, UI-only controls, and invalid combinations.
• Internal links and sitemap entries expose only the bounded set of indexable URLs.