Add GitHub Actions CI for Kani + proptest + fuzz verification

[avrabe]

Part of the V&V coverage initiative.

Problem

wohl has Kani harnesses, proptest suites, and a cargo-fuzz setup with targets — but no .github/workflows/ directory. The verification that runs locally is invisible in PRs and not gated on merge.

Acceptance

• .github/workflows/ci.yml on PR + main:
  • cargo fmt + clippy + test
  • Kani verification on existing harnesses
  • proptest suite
  • cargo-fuzz smoke (60s per target)
• Status badge on README
• Add a nightly long-fuzz job if the target mix warrants it

Notes

• Scope is sensor monitoring (alert/leak/temperature) — exercise threshold-crossing edge cases in proptest
• Keep it consistent with the rivet CI shape so assessors see a uniform pattern across the estate