Returns the stored .rdt bytes for a digest.

The response is application/octet-stream with Cache-Control: no-store. A malformed digest returns 422 (expected sha256:<64 hex>). An unknown digest returns 404.

The store only holds trajectories that passed admission. It lives in --dense-store-dir, else WELD_PLANNER_DENSE_STORE_DIR, else ~/.cache/rosieos-olp/weld-planner-dense, as sha256-<hex>.rdt. It is a cache: if a file is gone, plan again. OLP's Load fetches from this route and answers dense_blob_not_found when the planner no longer has the file. See the .rdt format for the file layout.