test(fuzz): the four-target fuzz sidecar (w5-fuzz-harness)

The family standing rule (anything worth fuzzing gets fuzzed) on the
crate's ingress surfaces, per the alkgit convention:

- fuzz_targets/{exec_parse,fingerprint,key_decode,terminal_params} —
  thin fuzz_target! wrappers over four ingress surfaces:
  - exec_parse: the production parse path (from_utf8-fallback ingress
    shape), differentially paired against a from-spec grammar oracle;
    typed routing classes + determinism.
  - fingerprint: blob parse + the single fingerprint path; the
    SHA256:/unpadded-base64 contract shape via a from-spec
    base64 round-trip oracle; the committed ssh-keygen -lf vectors
    replayed inside the gate.
  - key_decode: the production decode arm (the #[doc(hidden)]
    decode_key_arm exposure, routed through itself); typed-class +
    bounded-fixed-message invariants; passphrase-shape robustness
    drive on the underlying decode.
  - terminal_params: the pty_request conversion via the #[doc(hidden)]
    terminal_params exposure; exact clamp, verbatim term, the
    reserved-modes {} JSON shape, serialized-shape determinism.
- shared/ — the invariant logic as a stable-clean library; the
  corpus gate (cargo test --manifest-path fuzz/shared/Cargo.toml)
  replays every committed seed through the batteries and joins
  AGENTS.md's verification list.
- corpus/<target>/ committed seeds + the deterministic
  gen_fuzz_seeds.py (no-op-diff verified).
- run-detached.sh + nightly pin; the sidecar excluded from the root
  workspace (workspace + package excludes; publish dry-run clean).

Exposure shape decided + recorded per target (README + lib.rs doc
comments): fingerprint needs none (fully public ingress); the rest
are #[doc(hidden)] re-exports of the production surfaces, never
forks. Detached campaigns ran per target (600 s default budget, fork
mode): all four clean; the two findings of the first round are
recorded in the task file (the harness oracle's double-/-strip bug;
the PPK echo fixed in the preceding commit).

Verification: corpus gate green (11 tests); scripts/matrix.sh green;
taskgraph validate clean.

AGENTS.md

@@ -167,6 +167,8 @@ cargo publish --dry-run --allow-dirty               # before a release
 scripts/matrix.sh                                   # the decline-matrix runner: the full suite per
                                                     # feature build (none / git / session / tunnel /
                                                     # sftp / --all-features), clippy ×2, fmt, doc
+cargo test --manifest-path fuzz/shared/Cargo.toml   # the fuzz corpus gate: the committed seeds replay
+                                                    # the `fuzz/` sidecar's invariant batteries
 ```
 
 If feature flags are added, also run `cargo test --all-features` and