Safe invariants for racing a guide crop against RF-DETR

Observation (2026-09-08 UTC, source: COR-4082 implementation and independent code-review/test evidence in the Shield repository): a low-latency user-guided barcode lane can safely race RF-DETR without changing the recovery path when four invariants are explicit. Derive the bounded guide crop and full frame atomically from one camera draw; keep confirmation/submission tied to the full frame; run guide attempts through the existing global/per-request scheduler with a short internal budget; and keep any timed-out non-cancellable native validation counted against capacity until it physically settles. Parser-valid guide success may return immediately, while guide miss/failure falls through to the already-running detector and the unchanged decoder strategy order. This design preserves rollback through a default-off flag and the legacy raw-image contract. Still unproven until physical-device, deployed-staging, representative-concurrency p95, GPU-drain, and worst-case native-memory checks are run.