ci(deps): add Dependabot config to surface Rust security alerts

This repo had no .github/dependabot.yml, so Dependabot was not raising
PRs for outstanding HIGH/CRITICAL Rust crate vulnerabilities. Adopting
the corrected RSR template approach: cargo PRs enabled, grouped
minor/patch, security PRs flow normally.

Author: hyperpolymath

.github/dependabot.yml

@@ -0,0 +1,27 @@
+# SPDX-License-Identifier: PMPL-1.0-or-later
+# Dependabot configuration — adopts the corrected RSR template approach
+# (open-pull-requests-limit: 10 on cargo, grouped minor/patch).
+
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    groups:
+      actions:
+        patterns:
+          - "*"
+
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+    groups:
+      cargo:
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+          - "patch"