Make CI pip indexes explicit with safe diagnostics and sequential fallback

Dockerfile

@@ -31,7 +31,8 @@ RUN --mount=type=cache,id=frameflow-pip-py314-${TARGETARCH},target=/root/.cache/
     if [ -f /build-wheels/SHA256SUMS ]; then \
       (cd /build-wheels && sha256sum -c SHA256SUMS) \
       && pip install --no-index --find-links=/build-wheels -r requirements.lock; \
-    else pip install -r requirements.lock; fi
+    else python -c "import os; from urllib.parse import urlsplit; print('Docker build pip index host:', urlsplit(os.getenv('PIP_INDEX_URL', 'https://pypi.org/simple')).hostname, flush=True)" \
+      && pip install -r requirements.lock; fi
 COPY pigway_frame_flow ./pigway_frame_flow
 COPY vendor ./vendor
 ARG BUILD_COMMIT=development