Stability checkpoint — bin/cluster/test-all.sh batched cluster validation (MR !180)

After bin/cluster/demo/up.sh (GCP) or bin/cluster/ovh/up.sh (OVH)
brings a cluster up, this new script bundles 8-10 manual kubectl
commands into a single pass with a clear pass/fail table.

Layers : cluster (nodes / kube-system / DNS / SC) · app (mirador NS /
deployment / endpoints / Postgres STS) · liveness (/actuator/health
via port-forward) · observability (prom + grafana, optional).

Modes : --json (CI-friendly) · --quick (skip port-forward + obs).
Exit code 0/1 wireable into nightly smoke / pre-demo gate.