Decayfmt 0.1 implements x-parameterized on-disk mutation for .idcy and .tdcy files

The GitHub repository aravpanwar/decayfmt, featured in This Week in Rust #660, ships a Rust binary that encodes source images or text into .idcy<x> or .tdcy<x> containers. Encoding itself performs no corruption; each subsequent open overwrites the on-disk bytes first, then decodes the damaged state for display. No recovery path exists from the file alone because the mutation is deterministic and destructive.

Benchmarks in the repository demonstrate the effect: an x=3 image retains structure after three opens while x=10 collapses to noise after one. Text output shows only printable ASCII substitutions, producing readable gibberish rather than binary artifacts. The mechanism relies on Rust file I/O ownership to guarantee the write-before-read contract without external locks.

The format functions as an enforceable social contract rather than cryptographic protection, a distinction missed by initial coverage that treated it solely as novelty. It echoes documented studies of bit rot in archival media and self-modifying code patterns from early virus research, yet adds an explicit, tunable entropy dial absent from those precedents. Operational consequence is immediate: any production workflow must treat the decayfmt file as the sole authoritative copy or maintain external backups.

Future extensions could integrate the same mutation primitive into FUSE layers or language runtimes for automatic data lifetime policies.