ci: run tests on self-hosted arcp runner pool (spruce)

Org-level ephemeral runners (spruce-arcp-*) — one job per container,
fresh workspace each run. CodeQL and publish stay on GitHub-hosted.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: nficano

.github/workflows/test.yml

@@ -37,7 +37,7 @@ permissions:
 jobs:
   test:
     name: test (py${{ matrix.python-version }})
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux, arm64, arcp]
     strategy:
       fail-fast: false
       matrix: