refactor: extract DownloadService file I/O into adapter

[danielcopper]

Problem

DownloadService (py_modules/services/downloads.py) performs direct file I/O operations that belong in the adapter layer:

• ZIP extraction (zipfile.ZipFile, extractall())
• File cleanup (os.remove(), shutil.rmtree())
• Disk space checks (shutil.disk_usage())
• M3U file generation (open(), write)
• File replacement (os.replace(), os.makedirs())
• Temp file handling

This violates the architecture rule that services should not do I/O directly — I/O belongs in the adapter layer.

Proposed solution

Create a RomFileAdapter (or similar) that handles local ROM file operations:

• extract_zip(archive_path, dest_dir) -> list[str]
• disk_free(path) -> int
• write_m3u(path, entries: list[str])
• remove_file(path) / remove_dir(path)
• replace_file(src, dest)

Define a Protocol in services/protocols.py. Inject via bootstrap.py.

Priority

Low — architecture hygiene. No functional bugs.

Discovered during

Architecture validation audit during #197 Block 1 (slot deletion).

[danielcopper]

Audit #284 found additional file:line citations beyond the original list. Consolidating here so #287 can close as duplicate:

Confirmed I/O sites (py_modules/services/downloads.py):

• :138-139 — os.path.isfile(), os.remove() in _remove_tmp_file()
• :150-172 — os.path.isdir(), os.listdir(), os.walk() in tmp cleanup (_clean_rom_tmp_files, _clean_bios_tmp_files)
• :183-184 — open() + fcntl.flock() for queue-file locking
• :249-250 — os.makedirs(), shutil.disk_usage() for space checks
• :286-340 — os.walk(), os.replace(), path-traversal checks, atomic finalization
• :476 — open() for M3U file writing
• :504-513 — os.path.exists(), os.remove(), shutil.rmtree() cleanup

Test-mock smell (proxy for un-injected I/O, also from audit):

• tests/services/test_downloads.py:111, 160, 460, 493, 1000, 1042, 1086, 1127, 1226, 1397, 1903 — patch("shutil.disk_usage", ...) — disk space check needs an injected DiskSpaceChecker or fold into the DownloadFileAdapter.

Rolling #287 into this issue.