Add property/fuzz tests, Go -race and a coverage report to the gate #54

The gate (.github/workflows/gate.yml) runs rustfmt, clippy -D warnings, cargo test --workspace, gofmt, go vet and go test. It has no property/fuzz testing, no Go race detector and no coverage report. #52 (a panic and a silent duplicate-key corruption reachable through the public API) is exactly what a generator over Value finds in seconds.

Proposal

1. Property / differential fuzzing: arbitrary YAML/JSON → reader → canonical → digest in Rust, compared with the Go result on the same input (extends the existing conformance/differential/ corpus idea with generated inputs). Properties to assert: no panic for any input the reader accepts and for any Value constructible through the public API; canonical output is valid JSON with sorted unique keys; canonical(canonical(x)) == canonical(x); Rust digest == Go digest.
   • Rust: proptest (runs in the normal cargo test, fixed case count in CI) and/or cargo-fuzz targets run on a schedule.
   • Go: native testing.F fuzz targets plus a seed corpus shared with Rust.
2. Go -race: go test -race ./... in the gate.
3. Coverage report (cargo llvm-cov / go test -cover) published in the job summary first; add a floor only after seeing the baseline, and ratchet it up, never down.
4. Counterexamples found by the fuzzers are added to the differential corpus as vectors.

Depends on #52 so the first fuzz run is not red on a known bug; #53 (private fields) will shrink the fuzz surface further but is not a prerequisite.

Acceptance

• gate.yml runs the property tests and go test -race; a scheduled job runs the long fuzz targets.
• Reverting the fix of Canonicalization panics on a directly constructed BigInt and silently corrupts duplicate-key maps #52 makes the property tests fail (verified once, noted in the PR).