The first calculateRowColBackProject mixed two baselines in one image:
content-excluded residual (~0, regression baseline) inside the
back-projection coverage, full raw-level image-FPN outside it. Whole
lines fell outside the max_abs gate and the outlier-weighted average
degenerated to the content mean: vectors reached ~2300 counts (legacy
~340) and stamped junk bands at the coverage-window margins into all
per-sensor images (v013-ROWCOL-01 MERGED-DBG slices 141/408 etc).

Now the two baselines never mix: masked UM (blur(value*mask)/blur(mask))
high-passes the residual within coverage only; NaN-aware line averages
use covered pixels only (getLineAvgNaN); lines with under 10% coverage
(ROWCOL_BP_MIN_COVER) fall back to the pure legacy UM+gating estimate
per line (mergeLineProfiles). Console reports the line-fallback share.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>