PHLEGM v6 PET 4016-N firmware cache

PHLEGM does not bundle Commodore ROM images. When an accepted required image is
missing, PHLEGM may fetch the preferred ROM declared in ../firmware.config.phlegm directly
from the configured source URL. A downloaded image is accepted only if both its
exact byte size and CRC32 match the CFG declaration, then it is cached here.

You may still place legally obtained PET ROM images in this directory, or pass
--firmware-dir <path>. Existing accepted files are used before any download.

Required for a normal PET BASIC 4 boot:
  - BASIC B000 (4096 bytes; patched 901465-23 preferred, 901465-19 accepted)
  - BASIC C000 (4096 bytes)
  - BASIC D000 (4096 bytes)
  - 40-column normal-keyboard screen editor E000 (2048 bytes; 60 Hz preferred, 50 Hz accepted)
  - KERNAL F000 (4096 bytes)
  - character generator (2048 bytes; video-only, not CPU mapped)

Use the monitor command FIRMWARE to see loaded/missing slots and source URLs.
