Deterministic timestamps for GDELT bulk fallback fixtures

Problem

Three GDELT bulk-fallback tests failed in CI even though the production fallback was behaving correctly. Their mock export timestamp was fixed at 20260713110000, but the tests used the wall clock when evaluating the rolling window.

Symptoms

• The unit job on PR #5314 rejected the mocked bulk events with rolling bulk window contained no priority-country material-conflict events.
• The same three tests reproduced locally once the wall clock was more than 24 hours after the mocked export timestamp.

What Didn't Work

• Re-running the tests without changing the fixture did not help: the cutoff moves with Date.now().
• Changing the production stale-event filter would have hidden a valid safety check rather than fixing the test input.

Solution

Inject a deterministic clock into every test that supplies the static bulk export fixture:

const BULK_FIXTURE_NOW = Date.parse('2026-07-13T12:00:00Z');

const result = await fetchGdeltConflictEvents({
  now: () => BULK_FIXTURE_NOW,
  fetchBulkEvents: async () => ({
    exportTimestamp: '20260713110000',
    events: [{ id: 'gdelt-event-empty-doc', country: 'Sudan' }],
  }),
});

This change is in tests/conflict-gdelt.test.mjs; production code remains unchanged.

Why This Works

The rolling merge uses gdeltAddedAt when an event provides it, otherwise it falls back to the export timestamp. It then excludes entries older than its 24-hour cutoff (scripts/_conflict-gdelt-bulk.mjs). A fixed now keeps the intentionally static fixture inside that window, making the fallback-contract test independent of when CI runs.

Prevention

• Inject now whenever a test combines a static timestamp fixture with rolling-window or freshness logic.
• Keep expiry behavior in a separate test with an explicitly advanced injected clock; do not rely on wall-clock time to exercise it.

Related Issues

• PR #5314 contains the CI repair and passed all required checks.