test(enterprise): verify D3 priority lease

[Gracker]

Summary

• add the D3 enterprise dual-window regression for frontend lease proxy targets and TraceProcessorSqlWorker priority ordering
• verify P0 frontend timeline work runs before queued P1/P2 work after the currently running P2 task completes
• mark D1/D2/D3 and §0.6.4 complete in the enterprise checklist; D1/D2 were already covered by the existing window regression script and this PR aligns the README status

Verification

• cd backend && npx jest --runInBand --forceExit src/scripts/tests/verifyEnterpriseMultiTenantWindows.test.ts
• cd backend && npx jest --runInBand --forceExit src/services/tests/traceProcessorSqlWorker.test.ts src/routes/tests/traceProcessorProxyRoutes.test.ts
• cd backend && npm run typecheck
• git diff --check
• npm run verify:pr