[nanvix] E: Bake ninja and Cython into toolchain image

.nanvix/docker/Dockerfile

@@ -6,9 +6,28 @@
 
 FROM ghcr.io/nanvix/toolchain-gcc:sha-34a3641
 
+# Install the host Python plus the build helpers required by extension
+# modules that ship a meson or Cython build step (numpy, scipy, pandas,
+# ...):
+#
+#   - ninja  — meson's default backend; missing it makes every meson-based
+#              extension build fail immediately.
+#   - Cython — required by numpy 1.26.4's `numpy/_build_utils/tempita.py`
+#              .pyx.in code generation. Pinned `<3` for numpy 1.26.x
+#              compatibility; lift the pin when bumping numpy.
+#
+# We deliberately purge `/usr/include/python3.12` after the install. The
+# `python3-pip` / `ninja-build` apt packages transitively pull in
+# `libpython3.12-dev`, whose headers under `/usr/include/python3.12` would
+# otherwise be picked up by meson's regen step ahead of the Nanvix cross
+# sysroot headers and silently corrupt the cross-build.
 RUN apt-get update \
     && apt-get install -y --no-install-recommends \
         python3 \
         python3-dev \
+        python3-pip \
+        ninja-build \
+    && pip3 install --break-system-packages --no-cache-dir 'Cython<3' \
+    && rm -rf /usr/include/python3.12 \
     && rm -rf /var/lib/apt/lists/* \
     && ln -sf /usr/bin/python3 /opt/nanvix/bin/python3