Post-merge hardening: CSV LF, version label, deferred globals, SECURITY.md, CHANGELOG, dependabot (#16)

Five follow-ups from auditing #15: CSV LF prefix, runtime version label, deferred window-global scan, SECURITY.md threat model, CHANGELOG.md, dependabot.

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+    groups:
+      actions-minor:
+        update-types: ["minor", "patch"]