Make C++ indentation consistent (4 spaces, no tabs)

.editorconfig

@@ -0,0 +1,10 @@
+# EditorConfig helps editors and agents match project formatting defaults.
+# C++ indentation rules are also documented in
+# .github/instructions/cpp.instructions.md.
+
+root = true
+
+[*.{c,cc,cpp,h,hh,hpp}]
+indent_style = space
+indent_size = 4
+tab_width = 4

.github/workflows/ci_linux.yml

@@ -15,6 +15,12 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v6
 
+      # Fail fast on C++ indent drift (4 spaces, no tabs). Full-tree scan against
+      # the checkout — not Bazel runfiles — so we avoid per-package source filegroups.
+      # Unit helpers also run under `bazelisk test //python:cpp_indentation_test`.
+      - name: Check C++ indentation
+        run: python3 python/tests/cpp_indentation_test.py
+
       # 2️⃣ Free up disk space (LLVM toolchain ~4 GB extracted; runners ship ~14 GB free)
       - name: Free disk space
         uses: jlumbroso/free-disk-space@v1.3.1