Exercise production-equivalent fallback and measurements

[swernerx]

Summary

• exercise Go serving and Rust shadow execution at the same one-shot release child-process boundary
• make fallback, rollback, failure observation, reset, and Go-authority failure paths executable and CI-blocking
• publish comparable runtime, output, artifact, and per-child peak-RSS evidence while retaining Go authority
• update ADR-0020 and the migration contract with the selected boundary and remaining caveats

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
• ./internal/oxc_reference/run-rollout.sh --output docs/evidence/primitive-literal-rollout-2026-08-17.json
• go test ./...
• npx hereby build
• npx hereby check:format

Closes #53