//! trybuild UI suite (spec #1635: DQ-1/DQ-3). Every case pins exact rendered
//! stderr — message AND primary span.
//!
//! Workflow: after intentionally changing diagnostics, regenerate snapshots
//! with `TRYBUILD=overwrite cargo test -p formars-derive --test ui`, then
//! REVIEW every `.stderr` diff by hand before committing (DQ-1 quality gate).
//! Snapshots regenerate ONLY on the pinned gate toolchain 1.98.0 (see
//! `.github/workflows/ci.yml`); never per-toolchain.