Targeted spec diff of a version vs. its predecessor.

Returns a property-level before→after diff of this version's dashboard spec against the immediately preceding version (version-1) — only what changed, not the whole spec. Objects are diffed by key; arrays of spec elements (panels, pages) are matched by id so reorders don't register as changes. The first version (no predecessor) is reported as initial: true with the whole spec as one added subtree at the root path. The change list is capped; when the cap is exceeded truncated is true and totalChanges reports the full count. Read access follows the dashboard — requires dashboard:read plus the same per-resource read authorization as GET /v1/dashboards/{id}.