Native Spectrum media support files

SYSTEM.bin: original Miles Gordon Technology GDOS3, 6656 bytes, extracted
from copysys 3 in DISCPSYS.TAP. Source archive:
https://sam.speccy.cz/mgt_dos/disciple_system_tape_tap.zip

PLUS_SYS.bin: original Miles Gordon Technology G+DOS2, 6656 bytes, extracted
from CONFIG2_C in the +D system tape. Source archive:
https://sam.speccy.cz/mgt_dos/plus-d_system_tape_tzx.zip

These are manufacturer DOS binaries, not Boulder Dash code. media.py applies
GDOS's documented standalone SETUP values to the disk copy of SYSTEM only:
offset 0x029C=0 (network disabled), offset 0x02A3=1 (printer disabled).

opus_boot.bin: 256-byte standard Opus Discovery 40-track single-side geometry
and bootstrap sector, inherited from the previously verified Space1999 media
pipeline, originally extracted from a Z80to v1.4 image. No game-specific
snapshot loader or other-game code is included. Z80to by TomDDG, MIT license:
https://github.com/TomDDG/Z80to
License retained in Z80to-LICENSE.txt.

Filesystem references:
https://sam.speccy.cz/mgt_dos/plus-d_tech_guide_rev8c.txt
https://github.com/fuse-emulator/libspectrum/blob/master/plusd.c
https://github.com/0sAND1s/HCDisk (CFSOpus native file format)
https://worldofspectrum.net/pub/sinclair/books/m/MicrodriveAndInterface1Manual.html
https://rk.nvg.ntnu.no/sinclair/faq/fileform.html#MDR

The media.py writer is self-contained and needs no other project's files.
