D107 — Declarative lint contracts in map descriptors

Decision. A map may declare required_fields, additive required_fields.<Type>, and require_index_entry in _map.md. Lint reports a missing required field as an error and an incomplete curated index as a warning; malformed recognized entries are informational. The contract is optional, so no server-default vocabulary or schema is imposed. gate_check fails on the error, while validate and contradiction-only commit_gate retain their existing responsibilities.

Rationale. A descriptor is versioned, local to the map it governs, and uses the existing string/list frontmatter grammar. It gives deterministic, auditable enforcement without executable plugins, which would make lint arbitrary-code execution and undermine its no-LLM guarantee. Required field contracts belong to governance rather than the read-path validator so legacy KBs remain consumable. Domain-specific rules (for example, required section prose or naming conventions) remain KB skill policy: generalising them would require a rule language outside this data-plane contract. index_incomplete deliberately complements, rather than replaces, expanded_as_category: the former reports each missing curated entry; the latter detects a broader filesystem-taxonomy smell.