ci: build test-fast with -j1 to avoid cc1plus OOM in the 800m container

Root cause of the test-fast failures (confirmed via a pure-main probe PR that
failed identically): the fast job builds with -j$(nproc) inside an 800m-memory
container, and parallel Eigen+GoogleTest compilation OOM-kills cc1plus
("Killed signal terminated program cc1plus") non-deterministically — unrelated
to branch content (only constants.hpp reaches the fast target's header closure
on this branch, with trivial constexpr additions).

Serialise the fast build (-j1), mirroring the Pi-protection already applied to
the test-cgal job.  Fixes test-fast on main and on this PR.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/cpp-tests.yml

@@ -41,7 +41,11 @@ jobs:
         run: cmake -S code -B build -DCMAKE_BUILD_TYPE=Release
 
       - name: Build
-        run: nice -n 19 cmake --build build --target conformallab_tests -j$(nproc)
+        # Serial build (-j1): the 800m-limited container OOM-kills cc1plus when
+        # several Eigen+GoogleTest translation units compile in parallel
+        # (`-j$(nproc)`), failing test-fast non-deterministically regardless of
+        # branch content.  Mirrors the Pi-protection already used by test-cgal.
+        run: nice -n 19 cmake --build build --target conformallab_tests -j1
 
       - name: Run tests
         run: >