test: harden flaky restart and startup checks

[moolen]

Summary

• wait for periodic checkpoint tests to observe the post-rotation manifest state
• stop fast-restart integration from asserting on async segment-index migration timing
• retry port-forward reconnection after Spectre pod restarts in e2e

Verification

• go test -count=50 -run TestEngine_StartPeriodicCheckpointPersistsRestartableStateWithoutFlush ./internal/embeddedstore
• go test -count=50 -run TestEmbeddedRuntimeFastRestartServesTimelineImmediately ./tests/integration/api
• go test -v -count=1 -timeout 60m ./tests/e2e -run TestScenarioPodRestart
• go test -v -cover -count=1 -timeout 60m ./...
• cd ui && npm run test
• cd ui && npx playwright install chromium && npm run test:ct