Add diffvg.pth to ensure pydiffvg is importable

Dockerfile

@@ -40,7 +40,11 @@ RUN git clone --recursive https://github.com/BachiLi/diffvg.git /opt/diffvg && \
     sed -i 's/if (frame && (std::string) str(frame->f_code->co_name)/if (0 \&\& (std::string) str(frame->f_code->co_name)/g' pybind11/include/pybind11/pybind11.h && \
     sed -i 's/frame->f_code->co_argcount > 0) {/0) {/g' pybind11/include/pybind11/pybind11.h && \
     sed -i 's/frame->f_locals, PyTuple_GET_ITEM(frame->f_code->co_varnames, 0));/nullptr/g' pybind11/include/pybind11/pybind11.h && \
-    python setup.py install
+    python setup.py install && \
+    # Ensure pydiffvg is importable - install to site-packages
+    python -c "import sys; print(sys.path)" && \
+    # Add diffvg to system path explicitly
+    echo "/opt/diffvg" >> /usr/local/lib/python3.12/site-packages/diffvg_path.pth
 
 # Install wiregrad from source (Leimuhun/wiregrad - gradient/repulsion loss)
 RUN pip install --no-cache-dir "git+https://github.com/Leimuhun/wiregrad.git"