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]>

platform/vuln-scanner/namespace.yaml

@@ -0,0 +1,23 @@
+---
+# vuln-scanner — the CVE visibility scanner (R-VULN-01..09, ADR-0027, docs/17). Hand-authored platform
+# delivery tooling, not a catalog service: its cluster-wide read identity is something a
+# ServiceDefinition cannot express.
+#
+# gateway-access=shared opts this namespace in to attach the cve.<domain> HTTPRoute to the shared
+# Gateway; the route targets the per-pod oauth2-proxy sidecar (:4180), never the page directly.
+#
+# INTENTIONALLY NOT homelab.keithjackson.dev/app-namespace=true (the crafty/cloudflared/pm-agent
+# posture): every Kyverno app policy matches on that label, and the generated default-deny would then
+# be double-owned with the one this tree ships (networkpolicy.yaml), the 2026-09-30 drift loop. The
+# pod still carries the egress-class and auth-mode labels those policies would require.
+#
+# PodSecurity `restricted` IS enforced: nothing here needs any privilege.
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: vuln-scanner
+  labels:
+    app.kubernetes.io/name: vuln-scanner
+    gateway-access: "shared"
+    pod-security.kubernetes.io/enforce: restricted
+    pod-security.kubernetes.io/enforce-version: latest