Extracted from ../entities/heading.md.
- v0.1 — initial draft (Batch C2 — gap fill). Consolidates rules from business-rules.md §3.1 (BR-001, BR-003, BR-004), glossary §3 (Heading, Text Block), and domain-model Heading entry. Adds validation invariants, depth-cap semantics, three worked examples covering simple two-level structure, depth-cap rejection, and independent Item nesting. Phase: Alpha. Status: 🟡 Working.