[sksizer]

What

PR 3 of the D-0020 series (#261, #262): the Rust engine mirrors the mcVersion 2 declarative surface, keeping the two engines semantics-identical ahead of the shared-fixture v2 twins (PR 4).

• declarative/schema_v2.rs — the JSON Schema 2020-12 subset: properties/required (optional-by-default; required entries must name declared properties)/additionalProperties, minLength/maxLength/pattern + composable format (the runtime Schema::Format widened to {format, min, max, pattern}, issues emitted in Zod's chained order), minimum/maximum, type: integer, items/minItems/maxItems, [T,"null"] unions, validated const; per-node closed keys with the same three rejection classes as the TS side (v1-spelling migration hints, the 34-keyword outside-subset list, unknown key).
• declarative/body_v2.rs — additionalSections, the full minContains/maxContains occurrence recipe onto the repeatable machinery, hoisted nested sections, list items (checkbox | schema); v1 keys rejected with migration hints.
• Finding.hint — Option<String>, serde-skipped when absent so existing consumers and goldens see byte-identical output; nearest-enclosing description resolution threaded through the structure/content/frontmatter walks with contract-root fallback.
• v1 documents compile unchanged; the envelope dispatches on mcVersion; inline config contracts compile with their config's version.

Example (same document + contract as #262's demo, Rust side)

mcVersion: 2
kind: contract
description: root desc
frontmatter:
  type: object
  required: [id]
  properties:
    id: { type: string, pattern: '^D-\d{4}$', description: "the D-#### id" }
body:
  order: none
  sections:
    - section: Summary
      description: one-paragraph outcome

let contract = load_contract(yaml)?;               // v2 dispatch
let findings = contract.validate(doc, "t.md");
// frontmatter/type            hint: Some("the D-#### id")   ← field description
// structure/section-missing   hint: Some("one-paragraph outcome")
// (hint is None — and absent from JSON — wherever no description is in scope)

Gates

• cargo test -p markdown-contract-engine: 180 unit tests + corpus harness green (up from 149; shared-corpus goldens untouched — 51 fixtures / 8 ts-only skips unchanged).
• cargo clippy --all-targets -- -D warnings clean; cargo fmt --check clean; cargo check --workspace --all-targets clean (one test-literal hint: None touch in apps/desktop's engine map to keep the workspace compiling).
• Envelope tests that asserted "mcVersion 2 is rejected" now assert v3 rejection (inherent to the version-set change).

Notes: this crate has no text renderer (the runner returns Findings), so the human hint: line lives on the TS side only; JSON serialization carries the field here. v2 parity fixtures for the shared corpus land in PR 4 with the corpus migration.