Installing cargo-fuzz under a nightly directory override made rustix's
build script (a cargo-fuzz dependency) gate on `rustc_attrs`, which this
runner's nightly rejects as reserved-for-rustc. Install nightly without
overriding the directory default so `cargo install cargo-fuzz` still
builds under stable; only `fuzz build`/`fuzz run` opt into `+nightly`.
Add isolated cargo-fuzz/libFuzzer infrastructure for irtt-proto, fuzzing
decode_request, decode_open_reply, decode_echo_reply, Params::decode(_with_presence),
and the varint codec against arbitrary and structurally-derived wire input.
Each target's core invariant is that arbitrary bytes must never panic; a
small clean-room seed corpus (generated from irtt-proto's own encoders) and
a bounded nightly smoke CI workflow are included. The fuzz project is its
own isolated cargo workspace and does not affect the root workspace, its
lockfile, or any published crate's dependency graph.
Nils Andreas Svee