We implement a hierarchical fallback in
classify_signal() that attempts the parent deep path
and, on exception, drops to a fast frequency-based classifier.
On synthetic RF IQ, we inject realistic failure modes (load
error, shape mismatch, NaN input, timeout, class mismatch) at
probability pfail and report rescue rate and accuracy impacts.
The rescue adds < 1 ms median latency and corrects a majority
of failed inferences across modes.