feat(vuln): vuln-scanner platform manifests, Flux tier, alerts, L0/L2/L3 tests

Hand-authored platform/vuln-scanner (non-gating Flux tier, digest-pinned Trivy 0.76.1 and
python 3.14.9-alpine, read-only RBAC, per-pod oauth2-proxy, internal-only cve.<domain>),
five alerts with promtool tests, R-VULN-01..09 registry entries, and vuln-* L0 checks with
mutation self-tests. src/ and the bot workflow are integrated separately.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

clusters/base/kustomization.yaml

@@ -78,6 +78,10 @@ resources:
   # `wg-server-key` Secret, so in the gating `platform` tier an unfinished manual step would have
   # stalled every apps wave. Carries no SOPS secret, so neither overlay adds a decryption patch.
   - vpn-ingress.yaml     # Flux Kustomization -> ./platform/vpn-ingress (dependsOn platform-config)
+  # CVE visibility scanner (docs/17, ADR-0027) — POST-platform tier (dependsOn platform-config,
+  # wait:false): a failing scanner must never gate the platform, and nothing dependsOn it. Carries the
+  # oauth2-proxy credential SOPS file, so both overlays add a decryption patch by name.
+  - vuln-scanner.yaml    # Flux Kustomization -> ./platform/vuln-scanner (dependsOn platform-config)
   # Cloudflare Tunnel connector (north-south INGRESS from the Cloudflare edge) — POST-platform tier
   # (dependsOn platform-config, wait:false), and NOTHING dependsOn it. Non-gating is load-bearing,
   # as for vpn-ingress: the Deployment cannot become Ready until the operator enrols the tunnel