Prune and fix CI test suites

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 #5573 merged (ee7a4e906; Jaah Deploy green). test.yml already green since #5561. Unit-wave spare CPU block moved to the API suite (full gate −27s, A,B,A,B); pane-row click race fixed; sweeps found nothing to delete. Named: one unreproduced TaskPane vitest flake.