fix(fuzz): the crate was invisible to lint, dependabot, audit, deny, and osv

[forkwright]

fuzz/ was invisible to every repo-wide check. Five of them.

I found this while producing lint evidence for #377: a kanon lint . --all at origin/main returned
48 findings, and 15 were in fuzz/ — every one introduced when that crate landed in #428, earlier
tonight. Nothing was ever going to catch them.

Why nothing saw it

The crate declares its own empty [workspace] table so a nightly sanitizer build stays out of the
stable workspace. That is deliberate and correct. It also means every check keyed to the root
manifest resolves a graph that does not contain it:

check	why it missed fuzz/
cargo clippy --workspace / the CI gate	the root workspace excludes the directory
kanon lint	ran, and nobody had looked at its output
dependabot	directory: / only
cargo audit	reads one lockfile
cargo deny	follows the manifest it is given
osv-scanner	the step named --lockfile=Cargo.lock explicitly

So libfuzzer-sys, prost, tokio-util and bytes have had no vulnerability scanning and no
dependency updates since the crate was added. Three separate security jobs, each looking like it
covered the repository.

What changed

The lint debt is cleared — kanon lint . --all now reports zero findings under fuzz/, repo
total 48 → 33. Manifest sections follow the canonical order the rule encodes (workspace, package,
bin, dependencies, package.metadata), the single-key [dependencies.kerykeion] block folds into
[dependencies], and a LICENSE sits at this crate's own workspace root — which is what
MANIFEST/license-spdx-match was asking for, since [workspace] makes fuzz/ a root in its own
right and the repository LICENSE one directory up does not reach it.

One fix was worth more than the lint that prompted it. message_parse had an .expect() flagged
as library code; replacing it with panic! traded one rule for another, which is the tell that
neither was the fix. The property wanted is a message this parser produced decodes again to the same
bytes — one comparison, not a match arm plus an assertion. It is now a single assert_eq! over
Option<&[u8]>, which also states the decode-succeeds half the previous shape left implicit.

The coverage gap is closed at each site: a /fuzz dependabot entry, a second cargo deny run
with --manifest-path fuzz/Cargo.toml, a second cargo audit with --file fuzz/Cargo.lock, and
--lockfile=fuzz/Cargo.lock added to the osv step. Each is a second invocation rather than a widened
first one, because no flag on any of the three spans two workspaces — the comments say so at each
site, since "why is this run twice" is the obvious question and collapsing them is the obvious wrong
answer.

fuzz/Cargo.lock is now committed, because dependabot updates a binary crate by moving its lockfile
and cannot act without one. It also removes the untracked file any local cargo check in that
directory leaves behind.

Verification

• kanon lint . --all at this branch: zero findings under fuzz/.
• cargo +nightly check --target x86_64-unknown-linux-gnu in fuzz/: Finished, exit 0 — the
  reordered manifest and rewritten target still build.
• Both YAML files parse.

pre-push-verify.sh derives crates from the branch diff and finds none: this touches only fuzz/
(outside the workspace) and two CI files. The real verification is the security jobs themselves
reporting on this PR, which is the thing that could not happen before it.