Propose a template mapping onto a target dataset

Map a source v3 dashboard's field ROLES onto a target dataset: profile both datasets, score every source field against the target's columns, greedily assign one target field per role, and return the proposed mapping + a plain-language fidelity tally + the target's field lists (for the override pickers). Pure deterministic computation — no LLM, no writes — so it is synchronous (the profile builds run off the event loop). Auth: read on the source dashboard + the target dataview ACL.