Extracted from ../entities/unit.md.
- v0.1 — initial draft (Batch C2 — gap fill). Consolidates rules from business-rules.md §3.12 (BR-110, BR-111, BR-112, BR-113), glossary §9 (Unit), domain-model Unit entry. Adds validation invariants, built-in seed list, three worked examples covering built-in usage, custom Unit, and unit-mismatch anomaly. Phase: Alpha. Status: 🟢 Locked.