PR #805 review lesson (2026-09-08; source: e3-solutions/theft exact diff origin/staging…

PR #805 review lesson (2026-09-08; source: e3-solutions/theft exact diff origin/staging a72d82dd… to ff04495b…): starting RF-DETR concurrently is insufficient if downstream retained scheduling still awaits a guide-lane budget; that serializes detector-first successes. Also, loser cancellation for a shared non-preemptible detector must not reuse destructive timeout recovery, or normal fast-path wins can restart the sole runner and cause unrelated 503s. Optional client fast paths must treat allocation/encoding failure as a miss and still send the required full frame.