Preview a template mapping applied to a target dataset

The apply modal's live mapping-change preview: build the applied canvas for a target dataset (resolve the mapping when unconfirmed, else use the passed one + overrides), derive its plan/specs through dash-render, and return them in the SAME render-payload shape as /{id}/canvas alongside the target-resolved mapping + fidelity — so one call feeds both the canvas-host preview AND the per-role recipe. NOTHING persists: no dashboard row, no bake, no job — the plan is derived non-persisted (plan_only) and no data is resolved (the preview renders static, figures as skeletons; live data exists only after apply). Accepts the SAME inputs as /v3/templates/apply so the modal's state maps 1:1. Synchronous (called on every override change) — the profile builds + plan round-trip run off the event loop. Auth: read on the source dashboard (curated-template bypass) + the target dataview ACL (mirrors resolve-mapping).