//! Regression tests for the root workspace's `serde_json` feature
//! unification (report 08 F2).
//!
//! The root `Cargo.toml` pins `preserve_order` + `arbitrary_precision` on
//! `serde_json` specifically because `vendor/headroom`'s `headroom-core`
//! requires them and Cargo unifies features across a single dependency
//! graph - if the root workspace ever drops (or a `cargo upgrade` pass
//! regenerates) that feature list, headroom-core silently gets built
//! without them. That happened once already (see
//! `docs/HEADROOM-FORK-DIFF.md`'s 2026-07-11 merge section): object keys
//! came out alphabetically sorted instead of insertion-ordered, breaking
//! SmartCrusher's anchor matching, with no compile error and no other
//! failing test. These tests assert the actual behavior the two features
//! grant, so losing the features fails a test regardless of how they were
//! lost.