Explore dynamic pip package installation for pyhl

[danbugs]

Summary

Currently pyhl ships a fixed set of preloaded pip packages baked into the python-agent-driver CPIO. Investigate whether users can dynamically add pip packages without rebuilding the entire image.

Questions to explore

• Can we mount a host-side site-packages directory into the guest via --mount and have Python pick it up?
• Could we layer a second CPIO or overlay onto the initrd at runtime?
• Is there a way to run pip install inside the guest at warmup time (before snapshot) so new packages persist in the snapshot?
• What are the constraints (guest memory, filesystem model, snapshot size)?

May not be feasible given the cpiovfs + snapshot model, but worth investigating.