curt_synth_bg was fully parsed/persisted in IntersceneMatchParameters
but never read by the detector: the synthetic grid was always ADDED on
top of the LoG-conditioned real pyramid, so "Synthetic over real
background = false" had no effect (real background noise, and any NaN
border pixels, always present). Now when curt_synth_bg=false the level
is zeroed before the synthetic targets are injected -> truly clean,
noise-free targets. Logs which path was taken.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>