repo: publish the host regression suite; consolidate .gitignore

- tests/ (114 host cases over production sources), the CI workflow
  (ASan/UBSan on PRs and master pushes) and REGRESSION_TEST_PLAN.md
  are tracked
- .gitignore grouped and deduplicated (173 -> 96 lines, same ignored set)

.github/workflows/tests.yml

@@ -0,0 +1,32 @@
+name: host regression tests
+
+on:
+  pull_request:
+  push:
+    branches: [master]
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  quick:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.12'
+      - name: Install host build tools
+        run: sudo apt-get update && sudo apt-get install -y cmake ninja-build g++
+      - name: Run real production code with ASan and UBSan
+        run: python tests/run.py run --profile quick --sanitize
+      - name: Retain results and diagnostics
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: host-regression-results
+          path: test-results/
+          if-no-files-found: error
+          retention-days: 14