decmpfs — apply the operating system's transparent per-file compression to a file
in place: macOS APFS (decmpfs), Linux btrfs, Windows NTFS. The kernel decompresses
on read, so the file keeps its logical size + exact contents and loads at near-native
speed while taking less space on disk.
compress_file(path) detects the filesystem, applies compression, then verifies the
kernel reads the bytes back identically — rolling back on any failure. probe(path)
is the detect-only / capability-reporting half.
Backends: btrfs (FS_COMPR_FL + the btrfs.compression property), NTFS
(FSCTL_SET_COMPRESSION), and macOS decmpfs (resource fork, kernel-roundtrip
verified); other targets report Unsupported.
Contract: every Outcome is a SUCCESS; Err is reserved for genuine I/O failures
that leave the file's integrity unknown. An unsupported FS, a permission/lock issue,
an incompressible or too-large file are non-fatal Outcomes.
Panic-free invariant: the deny below keeps non-test code free of the obvious panic sources; all slice indexing is length-guarded.