building-a-format-package

Use when building, scaffolding, or extending an @effected format package — a parse/edit/format library for a text format (jsonc, yaml, toml, markdown, or the next one). Covers the canonical module-per-concept surface (facade/Document/Node/Edit/Format/Visitor/Diagnostic), the engine-origin policy (own the engine — vendor+harden or from-scratch, never a runtime parser dependency), the Effect-wrapping and Result-parity policy, the cross-package parity contract (Edit/Range field-identity, the diagnostic five-field core, MAX_NESTING_DEPTH=256), the conformance-corpus harness pattern (vendored fixtures + pin file, empty skip map, differential oracle), and the scaffold gotchas. Trigger phrases — new format package, corpus harness, VENDORED pin, differential oracle, parity contract, parse/edit/format surface.