ci: add CodeQL code scanning (C#)

.github/workflows/codeql.yml

@@ -0,0 +1,39 @@
+name: CodeQL
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  schedule:
+    - cron: '23 5 * * 1'   # weekly, Monday 05:23 UTC
+
+permissions:
+  contents: read
+
+jobs:
+  analyze:
+    name: Analyze (csharp)
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write   # upload analysis results to the Security tab
+      actions: read
+      contents: read
+    steps:
+      - name: Checkout
+        uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          persist-credentials: false
+
+      # build-mode: none analyzes the C# sources directly (no build infra needed, runs on ubuntu).
+      # Switch to a built mode on windows-latest later if more precision is wanted.
+      - name: Initialize CodeQL
+        uses: github/codeql-action/init@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
+        with:
+          languages: csharp
+          build-mode: none
+
+      - name: Perform CodeQL analysis
+        uses: github/codeql-action/analyze@8aad20d150bbac5944a9f9d289da16a4b0d87c1e # v4.36.2
+        with:
+          category: "/language:csharp"