POSE-13 forensics (from files, run 02:18-02:24): the iteration executed but dG
came out defined on only 5% of the frame (right-edge strip + terrain patches),
so fpn2 = fpn1 over 95% - the ghost survived. Chain: rebuild accumulation was
healthy (494/494 scenes, ~75% per scene, ~85% union) but a single-sequence
new-average center keeps NaN holes in the weak sky (no parent cumulative, no
fillNewGaps), and convert_direct of a NaN-holed center image NaN-poisons
nearly all back-projected tiles (task inversion + overlap spread).

Fix: composite before the second back-propagation - keep rebuilt values where
defined, fall back to the original center in the holes. There dG measures the
pass-1 residual (~0), so it degrades to "no correction where not rebuilt"
instead of NaN. Adds a coverage printout and a cuas_debug save of the
composited center (-FPN-CENTER2-ITERn) so the next run is file-verifiable.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>