chore: split validation and release workflows and refactor packaging

.github/workflows/validate.yml

@@ -0,0 +1,42 @@
+name: Validate
+
+on:
+  push:
+    branches: ['**']
+  pull_request:
+  workflow_call:
+
+permissions:
+  contents: read
+
+jobs:
+  validate:
+    name: Validate
+    runs-on: linux-amd64-docker-small
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: '1.25.x'
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Test workflow scripts
+        run: python -B -m unittest discover -s .github/scripts -p 'test_*.py'
+
+      - name: Verify snapshot
+        working-directory: tools
+        run: |
+          go run . upstream prepare
+          go run . upstream verify
+
+      - name: Tools test
+        working-directory: tools
+        run: go test ./...
+
+      - name: Build and render all themes and official languages
+        working-directory: tools
+        run: go run . preview all