Clean up and fix test suite failures

Review all test suites, delete stale, superseeded, those that were one off to check feature flags or absence of something. Kill the most long running or flaky with unit tests that execute in milliseconds. Regroup between quick and full, Fix broken tests in CI - GitHub Actions - look at a few recent test runs and fix broken/flaky tests. Babysit CI/CD until the tests (test.yaml) pass.

PR #7985 queued: fixed a real double-delivery race on legacy-gate answers (CI flake), 2 timing flakes, 1 stale test deleted, 3 quick census stages cheaper (4.4s→0.2s etc.). Develop test.yml baseline green; other red runs were already fixed or infra. Memory: none applied.