Deferred-correction interface fluxes for the adaptive grid

Two-point fluxes across coarse-fine faces miss the tangential potential
gradient (offset leaf centers), biasing R ~0.5-2% low. After the first
solve, per-leaf gradients are reconstructed by least squares over face
neighbors and the known tangential term g*delta*Gt moves to the
right-hand side of a re-solve (ADAPTIVE_CORRECTION_PASSES, default 1).
The matrix is unchanged, so the new PreparedSolver reuses the LU
factorization / AMG hierarchy across passes; the corrected currents
satisfy KCL exactly, so the power-balance identity, via currents and
part fluxes all use them consistently (edge power = dV * I_corr).

Measured: strip worst case -1.74% -> -0.028% (1 pass); feature-dense
plate end-to-end -1.1% -> -0.011% at 7.2 s vs 25.9 s uniform (5.58M ->
823k unknowns). Tests tightened accordingly plus a passes-knob test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

fill_resistance/config.py

@@ -68,6 +68,11 @@ ADAPTIVE_MAX_CELL_UM = 2000.0   # coarsest leaf edge length. The MINIMUM
                                 # caps leaf growth near features
 ADAPTIVE_GUARD = 4              # a leaf of size s needs >= GUARD*s cells of
                                 # clearance to the nearest feature
+ADAPTIVE_CORRECTION_PASSES = 1  # deferred-correction re-solves fixing the
+                                # coarse-fine interface flux bias (same
+                                # matrix, reused factorization/AMG). 1 pass
+                                # cuts the raw ~0.5-2% low bias to <0.03%
+                                # measured; 0 disables
 
 # --- Solver ---
 CONTACT_MODEL = "uniform"       # "uniform": conductor pressed on top injects