Diagnostic for the CORR2D-all-NaN numeric divergence: after the first
ref_scene=true convert, read back gpu_clt_ref (tp_proc_get_clt use_ref=1) and
print NaN%/nonzero/min/max once. Confirms whether the reference convert
populates gpu_clt_ref at all (vs scene gpu_clt which is correct -> SOURCE).

Prints one "PROBE gpu_clt_ref[cam0]: ..." line to System.out (captured in the
per-scene -SYSTEM_OUT.log). TEMP — remove after the divergence is fixed.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>