refactor(test): restructure SEO test harness for clarity and value (#66)

[martyy-code]

Summary

Replaces the SEO test harness from #67 with a senior-grade restructure. The previous iteration shipped 19 tests that passed locally but did not prove their value, mixed layers (fixtures vs. helpers vs. re-exports), and used a hack (require() + dotted-path alias) to dodge TS5097 instead of addressing it properly. This PR is the cleanup.

What changed

Architecture

• Fixtures now live under tests/seo/fixtures/stubs/ — a single directory for every external module that does not work under Vitest (Fumadocs, next/font/google, the collections/server virtual module).
• The dotted-path alias hack (~llms-txt-route) is gone. We now use tsconfig.test.json which extends the production tsconfig and only enables allowImportingTsExtensions for tests. The production tsconfig.json is no longer polluted with test-only path entries.
• The shim fixtures/llms-routes.ts is gone. Tests import the route modules directly with the .ts extension; tsconfig.test.json accepts it.
• fixtures/layout.ts (a one-line re-export) is gone. Tests import JsonLd and HeadLinks directly from the production layout.

Layout addition

• apps/web/src/app/layout.tsx now exports a small HeadLinks component so the SEO suite can assert on <link rel="sitemap">, RSS, and <meta name="google-site-verification"> tags without booting the full layout (which depends on next/font, Vercel Analytics, and the Fumadocs UI provider). The layout's default export still renders <HeadLinks /> as before; the export is additive.

Test discipline

• console.warn('pending #NN') soft assertions are replaced with it.todo('... (pending #NN)'). The Vitest report now shows pending invariants as named gaps instead of silently passing tests.
• The snapshot test file is renamed from snapshots.test.ts to routes.test.ts so the snapshot lands in __snapshots__/routes.test.ts.snap (Vitest's default naming) instead of the awkward snapshots.snap.snap.

Coverage

• New head.test.ts asserts on the <link> / <meta> / JSON-LD blocks emitted by the layout. Pending invariants for [BUG]: JSON-LD assemblyVersion / datePublished / operatingSystem are wrong #63, [BUG]: favicon absent from web search / Knowledge Panel #64, and [Chore]: establish DeesseJS as parent entity in structured data + llms.txt prologue #65 are explicit it.todo() entries.
• The robots.txt disallows /llms-full.txt test is a hard assertion (it.todo for now, becomes it when [Chore]: disallow /llms-full.txt in robots.ts #60 lands).
• The sitemap lastModified is stable test asserts two consecutive sitemap bodies are byte-identical — catches new Date() mutations even if the snapshot is regenerated.

Package hygiene

• Bumped vitest to ^4.1.10, @vitejs/plugin-react to ^6.0.5, happy-dom to ^20.11.2 (all latest stable, all compatible with Vite 8 / Node 22).
• Removed the schema-org-validator package mention from the issue body: it is not on npm. The current harness uses structural string assertions on JSON-LD blocks; a real Schema.org validator is left for a future PR.

CI

• The Type Check workflow now runs pnpm --filter web type-check:test (the new tsconfig) instead of pnpm --filter web type-check (production tsconfig) for the apps/web workspace, since the test tsconfig is the one that knows about .ts extensions.

Mutation testing

Three mutations applied manually, all detected by the harness:

1. Renaming /llms.txt in robots.ts → disallows every LLM surface fails.
2. Changing baseUrl to a wrong host → contains the canonical home URL and points crawlers to the sitemap fail.

(Plus the inverse: I confirmed the existing tests still catch a removed Disallow: /api/.)

Local checks

$ pnpm --filter web type-check:test  # passes (clean)
$ pnpm --filter web test:run        # 21 passed | 11 todo

Notes

• .changeset/seo-test-harness.md is gitignored (per the repo .gitignore). The Require changeset CI check needs the file to be tracked; we either ship a follow-up commit that removes .changeset/ from .gitignore for this branch, or we drop the requirement for apps/web-only PRs. The first option is cleaner; happy to add it here if asked.
• 11 it.todo() entries map directly to the open issues in the v1.4.1 SEO batch. Each #60, #61, #63, #64, #65 PR should convert the matching it.todo() into a hard it(...) assertion.