pi_img_diskforge — real image<->files for pi-img

pi_img_diskforge.py sits beside server.py (server.py UNTOUCHED) and exposes list_files() / extract_files() that walk a disk image guest filesystem in usermode (no mount/loopback/qemu-nbd/kernel) via the diskforge reader — the image<->files capability pi-img""s REUSE.md advertised but never implemented.

Verified: extract_files on a real ext4/MBR image wrote 8 files -> 4 distinct blobs (dedup); stock server.py still imports. Requires diskforge on PYTHONPATH; inert otherwise. Lives on branch ticket/494 (commit a41d5c7). Tracked by congruency #494. Depends on diskforge (#495).