Read trailer plan coverage.

Returns the pre-render report for the exact beat plan this trailer would render right now: whether it still describes the current plan, the 0–10 score, the 7.0 gate and every reason it withheld a pass, seven dimensions, machine-checked plan defects, per-beat notes, and priority fixes. While the latest draft edit's coverage waits to start it answers status: "queued"; a plan nobody has graded yet answers status: "missing" with the reason — NOT a 404, which would read as "no such trailer". This never returns storyboard boards. Scope: story:read.