fix: add self-canonicals for sitemap routes

[sarthakagrawal927]

Closes #86

Summary

Seven public sitemap URLs either lacked a self-canonical or pointed at extension-suffixed variants. This makes PUBLIC_CANONICALS the single source of truth for every public sitemap route's exact, extensionless self-canonical and adds a contract test that prevents future drift.

Changes

• src/lib/public-canonicals.ts — registers all nine public sitemap routes (home, discover, project-preview, tools, catalog-updates, changelog, about, privacy, terms) with exact self-canonical paths; exports PUBLIC_CANONICAL_PATHS.
• src/app/sitemap.ts — builds sitemap URLs from PUBLIC_CANONICALS so the sitemap and canonical cannot drift; the homepage entry now matches its canonical trailing slash.
• src/app/page.tsx — adds a homepage self-canonical (/) to the Next.js fallback page metadata (production home is the Astro overlay, whose canonical is already emitted by the Astro layout).
• src/app/about/page.tsx / src/app/catalog-updates/page.tsx — route their alternates.canonical through PUBLIC_CANONICALS instead of literal strings.
• src/__tests__/sitemap.test.ts — replaced with a sitemap/canonical contract test asserting:
  • sitemap advertises only the real public routes in canonical order;
  • full parity: sitemap pathnames === registered canonicals (no drift either way);
  • every canonical is extensionless and self-referential;
  • the seven named public surfaces (home, discover, project-preview, tools, changelog, privacy, terms) are registered;
  • canonicalPath maps the Astro .html overlays (index.html, changelog.html, …) to the registered extensionless canonicals.

Notes

• The Astro-served home and changelog pages already emit correct self-canonicals via landing-astro/src/layouts/Layout.astro + canonicalPath; this formalizes them in the registry and pins them with the contract test.
• No product behavior or visible UI changes. Routes and navigation are preserved.

Verification

pnpm quality   # check + typecheck + test:coverage + docs:check + code-health gates — green (215 tests)