FPN = mean(scene - backProject(center)) is a fixed point that preserves the
center's own content residual (the rotating ghost): recomputing against the
same contaminated center reproduces the same ghost regardless of rot_period
or photometric vintage (POSE-12 confirmed: 43.1/92.0 unchanged).

Iterate per Andrey's design (internal handoff 2026-07-06):
  1. subtract fpn_k from scenes in memory (setApplyFPN, delta-aware)
  2. rebuild the center IMAGE from the corrected scenes over integer
     rotations - new Cuas.rebuildCenterImageInMemory(): new-average only
     (no parent cumulative fold), NOTHING persisted, center_clt and
     image_center untouched (the .cuas cumulative is upstream state)
  3. back-propagate against the rebuilt center image (swapped in/restored
     around the measurement) -> residual dG = the ghost
  4. fpn_{k+1} = fpn_k + dG in NEW arrays (scenes hold fpn_k references as
     applied-FPN for delta re-apply); prints |dG| RMS/max per iteration
  5. cuas_debug saves per-iteration -FPN-DG-ITERn

Gated by new dialog field "FPN back-propagation iterations" next to
"Rotation period"; default 1 keeps the POSE-12 baseline bit-reproducible.
Verify by RT-RENDER horizon/sky pk-pk (43.1/92.0 -> temporal-noise floor).

Data path verified: applyFPN sets hasNewImageData, honored by both
GpuQuad.setBayerImages and GpuQuadJna, so corrected pixels reach the GPU
for the rebuild and the second back-propagation.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>