Rewrite the root README so a new contributor can understand the concrete problem, users, inputs, outputs, golden case, product boundary, reuse-first composition and staged roadmap without reconstructing the vision from Phase 0 evidence documents.
Out of scope: product implementation, final schemas or APIs, dependency changes, edits outside data2dsl, and changes to technical decisions already recorded in evidence documents.
The root README is now the canonical onboarding description of the planned product. It distinguishes intended behavior from unimplemented contracts and links detailed capability evidence rather than duplicating it.