test: assert every declared e2e fixture build is visited by a spec

[hivecommons-hive]

Test Improvement

Adds one case to tests/e2e-data-fixture-integrity.test.mjs: every name in
coverageBuildNames() must appear as a route in at least one tests/e2e/*.spec.js.

A fixture build exists because tests/e2e/fixtures/data-<name>/ exists, and each one
is a full Docusaurus compile (tests/tools/e2e-coverage-builds.mjs), so the
end-to-end coverage job's wall time is linear in those directories. The four cases
already in this file hold every committed overlay to properties a stale fixture would
break; none of them asks whether a browser ever opens the site compiled from it. A
build no spec visits is served, never requested, and produces no capture artifact, so
tests/tools/e2e-coverage-report.mjs has nothing of that build's to union and the arm
its overlays hold open goes back to being uncovered — at the price of a compile on
every run.

Scope: tests/e2e-data-fixture-integrity.test.mjs only. No production code, no
package.json thresholds, no workflow files, no change to any existing case.

Verification

• TZ=UTC node --test tests/e2e-data-fixture-integrity.test.mjs — 6 pass, 0 fail.
• Checked against a planted failure, not only against green: copying one overlay into
  tests/e2e/fixtures/data-unvisited/ makes the new case — and only the new case —
  fail naming that directory, while the four existing overlay cases stay green.
• npm run test:unit:coverage:check (node v26.10.0, TZ=UTC) exits 0:
  src files | 100.00 | 100.00, harness files | 100.00 | 99.19,
  all files | 99.65 | 96.13.
• npx prettier --check clean on the edited file.

Related Issue

Closes #1259

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Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88

[hivecommons-hive]

⚠️ The hold label did not get applied when this PR was opened: GitHub returned 504 on POST /repos/cncf/endusers/issues/1260/labels, and a retry through the relay reports the operation needs reconciliation before it can be retried. Agents cannot set hold themselves — it is a hive-controlled label.

This PR is hold-gated and must not be auto-merged. Please apply hold (and quality, testing) manually. Nothing about the diff is affected: it is one new test case in tests/e2e-data-fixture-integrity.test.mjs.

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🐝 Hive Agent: quality | Instance: hosted-available-lke648397-260827-5n31 | SHA: 28225fd

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88