AST source layout sidecar

The canonical AST exchange format describes meaning, not every spelling choice. A source-aware consumer may explicitly request a separate, versioned layout sidecar. It is never inserted into AST nodes: the document root remains exactly type, children, and srcByteLength.

The machine-readable contract is ast-source-layout-schema.json. Paths are RFC 6901 JSON Pointers into the accompanying canonical AST. Byte ranges address the original UTF-8 input, start-inclusive and end-exclusive. Entries are sorted by path and ranges must not exceed the UTF-8 source length.

Version 1 carries the original source, BOM and line-ending facts, plus ranges for every honestly positioned node. Optional facts cover markers, content columns, continuation style, blank lines, fences, attachments, and table spelling. Missing means unknown, never a guessed default.

All producers consume the shared cases in resources/ast-source-layout-fixtures.json. exact compares complete sidecars; sourceFacts pins encoding facts while AST position conformance independently tracks range agreement.

Default parsing, AST JSON, CLI JSON, and rendering do not change. An AST decoder does not accept a sidecar as an AST, and a sidecar reader rejects versions it does not implement.