Define the MAP architecture decision schema

[rajanbor]

Outcome

• adds a strict draft 2020-12 schema for ADR frontmatter
• reuses shared document field definitions through offline $ref resolution
• documents lifecycle, required fields, Markdown sections, migration, and rollback
• validates accepted and superseded fixtures, including conditional supersededBy
• validates real .map/decisions documents and detects dangling ADR references
• updates the generated ADR template with owners and tags

Verification

• node library/scripts/validate-schemas.ts (3 contracts)
• node library/scripts/build-registry.ts --check
• pnpm lint
• pnpm typecheck
• pnpm test (140 tests)
• pnpm build
• pnpm audit --audit-level=low
• map optimize --check (17% of budget)

Closes #91