Add cargo-fuzz targets for AADL parser + scheduling solver inputs

[avrabe]

Part of the V&V coverage initiative.

Problem

spar has proptest coverage on the parser but no cargo-fuzz directory. Adversarial AADL inputs and pathological solver inputs are exactly the robustness story fuzzing owns, and provide corpus for the criterion benchmark issue (worst-case inputs).

Recognized under DO-178C §6.4.2.2 (random testing) and ISO 26262-6 Table 9 (fault injection adjacent).

Acceptance

• fuzz/ directory scaffolded with cargo-fuzz init
• Fuzz targets:
  • fuzz_aadl_parse — malformed AADL rejected cleanly
  • fuzz_scheduler_solver — adversarial task sets do not panic / hang solver
  • fuzz_codegen_roundtrip — every valid schedule round-trips through codegen
• CI PR smoke: 60s per target
• Nightly: 1h per target, corpus persisted
• Corpus under fuzz/corpus/<target>/, reused by criterion benches for worst-case timing
• Traceability in rivet.yaml

Notes

• Libfuzzer-sys layout from relay/fuzz/
• Solver fuzzing will surface non-termination bugs — use -timeout aggressively