test(proto): add decoder fuzzing

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.

fuzz/.gitignore

@@ -0,0 +1,10 @@
+target
+artifacts
+coverage
+
+# The committed seed corpus under corpus/ is small and curated by hand; a
+# local `cargo fuzz run` grows each target's corpus directory far beyond
+# that, so ignore anything not already tracked instead of the directory
+# itself.
+corpus/*/*
+!corpus/*/