test: hold every committed e2e data overlay to the directory, not a list

[hivecommons-hive]

Test Improvement

Nothing in the repository reads tests/e2e/fixtures/data/ or
tests/e2e/fixtures/data-variants/ to find out what is committed there. The
two guards over committed overlays in tests/e2e-data-fixtures.test.mjs carry
hand-written file lists (lines 180-196 and 218-248), and
grep -n readdir tests/e2e-data-fixtures.test.mjs returns nothing.

applyOverlay is already strict about paths, so an overlay naming a field a
regenerated data file no longer has throws and npm run build:e2e:coverage
fails loudly. The failure mode with no symptom is the opposite one — an
overlay that applies cleanly and changes nothing:

• a set writing the value the data already carries;
• an append of a record a collector has since started producing itself.

The build succeeds, the coverage build compiles a document identical to the
real data, and the branch the overlay exists to reach silently stops being
reached. From CI that is indistinguishable from a branch that was never
covered.

This adds tests/e2e-data-fixture-integrity.test.mjs, which derives its cases
from the directory contents and holds every committed overlay to the weaker
property the directory alone can establish:

1. both fixture directories hold at least one overlay (so the sweeps below
   cannot pass vacuously);
2. every overlay names a data/<name>.json that exists;
3. every overlay still applies to it — the build's own guard, run where the
   message names the overlay instead of arriving from inside a webpack loader;
4. every overlay still changes the document it patches;
5. every overlay's description names the source file whose arm it covers, so
   a stale overlay is traceable to the component that stopped needing it.

It deliberately does not duplicate the existing guards: those pin
applyOverlay's behaviour against overlay documents built inline, which a
directory sweep does not and should not do.

Verification

Run locally at 8afaa67:

• npm run check — exit 0.
• npm run test:unit — 1946 passed, 0 failed (1941 before, plus the five
  cases here).
• npm run test:unit:coverage — the new file reports 100.00% lines and
  100.00% regions; src files unchanged at 100.00% / 99.92%.
• Negative control, isolated. Replacing
  tests/e2e/fixtures/data-variants/awards.json with a no-op that sets
  verifiedAt to the value data/awards.json already carries leaves the four
  other cases green and fails exactly one:
  tests/e2e/fixtures/data-variants/awards.json: applies cleanly but patches nothing, so the branch it exists to reach is no longer reached. The same
  no-op leaves npm run test:unit green without this file. The probe was
  reverted; it is not part of the diff.

Files claimed

tests/e2e-data-fixture-integrity.test.mjs (new). Nothing else is touched.
Disjoint from every open hold-gated PR — in particular it does not edit
tests/e2e-data-fixtures.test.mjs (#1034 lines 178-215, #1078 lines 78-91),
tests/e2e/data-variants.spec.js (#1034), or
tests/tools/e2e-coverage-report.mjs (#1040, #1070). It adds no fixture, so it
does not collide with #1093's data-variants/metrics.json, #1065's
fixtures/data/case-studies.json, #1054's fixtures/data/community-people.json,
or #1098's data-variants/radar-reports.json.

Related Issue

Closes #1099

Folding the two hand-written per-branch lists into the directory sweep does
require editing the occupied tests/e2e-data-fixtures.test.mjs; that is
recorded in #1099's Coordination section as out of scope and left for once
#1034 and #1078 land. It is not part of what #1099 tracks, so merging this
leaves nothing open on it.

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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]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.