feat(oxc-reference): add Go-vs-Rust semantic conformance gate

[swernerx]

Summary

• run the complete semantic corpus through the real Go tsfacts oracle and the Rust primitive/literal-v1 shadow candidate
• compare fact identity, all five type-view roots, response-local graph identity, structured primitive/literal/union payloads, diagnostics, states, and truncation with deterministic JSON diffs
• classify entries as semantic, transport, mapping, unsupported, or budget; fail CI on unexplained semantic/transport mismatches while publishing the full report as an artifact
• record the exact compatibility threshold and independent-producer replacement checklist in ADR-0018 and the RFC/migration docs

Boundary

This remains a shadow comparison. The Go checker stays authoritative, the Rust candidate still projects the Go-produced graph, and the TS7 producer protocol and production routing are unchanged.

Current gate result

• 6 corpus cases, 25 facts
• 125/125 ordered type-view roots preserve identity
• 7/7 complete in-category records match (1,000,000 ppm)
• semantic differences: 0
• transport differences: 0
• expected unsupported/error observations: 12
• expected budget/truncation observations: 46
• separately reported recovery-file mapping differences: 3
• blocking differences: 0
• two complete report runs were byte-identical

Validation

• cargo fmt --manifest-path internal/oxc_reference/Cargo.toml --all -- --check
• cargo clippy --locked --manifest-path internal/oxc_reference/Cargo.toml --all-targets -- -D warnings
• cargo test --locked --manifest-path internal/oxc_reference/Cargo.toml — 14 passed
• ./internal/oxc_reference/run-conformance.sh --output /tmp/ts7-rust-conformance-final.json — passed
• gofmt -d cmd/tsfacts/*.go internal/semanticfacts/*.go internal/tsfacts/*.go — no diff
• go test -count=1 ./... — passed
• go build -tags=noembed -o /tmp/tsfacts-issue41 ./cmd/tsfacts
• npx hereby lint — 0 issues
• npx hereby check:format
• npx hereby build
• git diff --cached --check

Closes #41