Rust unit testing

Use this guidance when the issue concerns structuring Rust tests so failures clearly explain defects, rather than how to run cargo test.

Working stance

• Keep setup, query, and assertion as separate concerns.
• Prefer pure query helpers that return data over helpers that panic halfway through the inspection.
• Make fallible setup explicit with Result and unwrap only at the test boundary with useful context.
• Use table tests for behaviour matrices, not for unrelated stories forced into one function.
• Serialize only tests that touch global state; otherwise let Cargo run them in parallel.
• Pick the assertion tool that makes the failing value easiest to inspect.

Test shape