merge: alert when a backup schedule has NEVER run, and when one is paused

VeleroNoRecentBackup could not see the failure that actually happened:
velero-daily sat paused having never produced a successful backup, so
velero_backup_last_successful_timestamp{schedule=...} did not exist and
time() - <absent series> yields no series. Prod had no volume backups and
nothing alerted, further masked by a resources-only schedule succeeding
daily. No velero_* metric describes a Schedule OBJECT, proven by scraping a
throwaway paused Schedule and finding zero lines, so this exposes the CRs
via kube-state-metrics customresource_state and keys on
defaultVolumesToFsBackup — the property, not a name list. Adds the repo's
first prometheus rule-unit-test harness, mutation-checked both ways.

ci/README.md

@@ -22,6 +22,7 @@ Individual stages:
 make -C ci lint        # yaml/lint                          (no cluster; live now)
 make -C ci l0          # schema + Kyverno + L0 static conformance (no cluster; live now)
 make -C ci l1          # L1 rendering checks                 (no cluster; live now)
+make -C ci prom-rules  # promtool rule-unit tests on alerts  (no cluster; live now)
 make -C ci conformance # L2/L3 in-cluster Chainsaw Job       (SKIPS unless CLUSTER_AVAILABLE=true)
 make -C ci test        # unit tests for these scripts themselves (ci/tests/*.test.sh)
 ```
@@ -102,13 +103,16 @@ in that case — nothing goes untested — and says plainly that the *scoped* ch
 
 ```
 ci/
-  Makefile                 targets: lint · l0 · l1 · conformance · gate · touched · test · tools · versions
+  Makefile                 targets: lint · l0 · l1 · prom-rules · conformance · gate · touched · test · tools · versions
   versions.env             PINNED tool versions (R-TEST-02) — single source of truth
   lib.sh                   shared helpers: logging, tool checks, cluster detection, artifact paths
   yamllint.yaml            lint config for the yaml/lint stage
   lint.sh                  Stage: yaml/lint
   l0.sh                    Stage: schema validation + Kyverno L0 gate + L0 static conformance
   l1.sh                    Stage: L1 rendering checks (Chainsaw L1 via the conformance runner)
+  prom-rules.sh            Stage: `promtool test rules` over platform/monitoring's PrometheusRules
+                           (harness: platform/monitoring/tests/). Catches the two alerting bugs no
+                           other stage can see — permanently-firing and structurally INERT rules.
   conformance.sh           Stage: L2/L3 in-cluster Chainsaw Job (trigger + collect)
   touched-requirements.sh  compute the R-* subset a change touches (drives the R-TEST-03 filter);
                            exits 3 LOUDLY rather than guessing when the diff base is unreliable