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/dialog.py

@@ -75,7 +75,7 @@ class _Dialog(QDialog):
 
         self.adaptive_check = QCheckBox(
             "adaptive cells (coarsen plane interiors; faster on large "
-            "boards, ~0.5–2 % low bias)")
+            "boards, corrected to ≲0.1 % of the uniform grid)")
         self.adaptive_check.setChecked(config.ADAPTIVE_CELLS)
         form.addRow("Grid:", self.adaptive_check)