Fix 13 issues from code review: source scoping, scalar refs, formula, ambiguities

Rewrite spec as implementation-ready v1, resolve all design questions

Note that preprocessing/ETL is out of scope

Add plain text format, file_hash provenance, text handling section

Expand _source into full _meta provenance block

Add source/ingestion section: JSONL, JSON, YAML, markdown formats

Add use cases section to README

Add DSL specification, examples, and project scaffolding

Initial commit