Add GitHub Actions CI for 25 Bazel test targets + fuzz + differential + Rocq/Lean proofs

[avrabe]

Part of the V&V coverage initiative.

Problem

relay has rich V&V content — 25 Bazel test targets, Rocq proofs in proofs/rocq/, Lean proofs in proofs/lean/ (BackpressureSafety, WcetAnalysis, CompositionalWcet), fuzz targets in fuzz/fuzz_targets/, differential testing in crates/relay-lc-diff/ — but zero .github/workflows/. Everything runs locally only.

Acceptance

• .github/workflows/ci.yml on PR + main:
  • bazel test //...
  • Proof jobs: bazel test //proofs/rocq/... and //proofs/lean/...
  • cargo fmt + clippy + test + doc
  • proptest suite
  • cargo-fuzz smoke (60s per target)
  • Differential tests (relay-lc-diff)
  • rivet validate
• .github/workflows/nightly.yml: long-fuzz runs (1h per target)
• Status badges: Rust, Rocq, Lean, Fuzz, Rivet
• Artifacts: proof logs, fuzz corpus, differential reports

Notes

• Rocq toolchain from rules_rocq_rust (Nix-hermetic); mirror meld's CI
• Lean toolchain from rules_lean, pin 4.27.0
• Keep engines (LC/SCH/SC/HS/CFDP) as separate bench/fuzz groups for targeted regression