Wire the adaptive quadtree grid into the solve path (phase 2)

config.ADAPTIVE_CELLS (dialog checkbox "adaptive cells", off by
default; standalone --adaptive) routes run_solve through
fill_resistance/adaptive.py: per-layer balanced leaf grids where every
non-uniform fine cell (electrodes, 1D chain cells, buildup, via-mouth
thickness map) is pinned at the fine size, leaf faces via the
series-half-cell rule, chain links and barrels re-attached by node id,
connectivity restriction and both contact models on the leaf graph via
solver cores extracted for reuse (_equipotential_core, _uniform_core,
_conductance_params, _barrel_links). All fields (V, |J|, power density)
are computed per leaf and expanded to the fine grid, so plots, summary
and dumps are unchanged.

Element sizes: minimum = the grid cell size itself (auto / dialog /
CELL_UM_OVERRIDE); maximum = ADAPTIVE_MAX_CELL_UM (2 mm default);
ADAPTIVE_GUARD sets the clearance a block needs to grow.

Measured end-to-end (feature-dense 120x120 plate, h=50um): 25.9 s ->
5.4 s, 5.58M -> 823k unknowns, R -1.1%. Accuracy documented honestly:
coarse-fine interfaces carry a first-order tangential flux error
biasing R low by ~0.5-2% depending on geometry (worst on narrow
strips); the earlier assumption that linear fields solve exactly on the
leaf graph was wrong - offset centers across size transitions leave an
unpaired residue. Gradient-corrected interface fluxes remain as phase 4
if tighter accuracy per leaf is needed.

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

README.md

@@ -137,6 +137,17 @@ SWIG API. Requires KiCad **10.0.1+**.
   AMG-preconditioned CG (pyamg) above — Jacobi-CG if pyamg is missing.
   Discretization error typically ≲ 2 % at defaults — halve the cell size
   and compare to judge convergence.
+- **Adaptive cells** (dialog checkbox, off by default; `ADAPTIVE_CELLS`):
+  solves on a 2:1-balanced quadtree — fine cells at copper boundaries,
+  electrodes, traces, via mouths and buildup, blocks up to
+  `ADAPTIVE_MAX_CELL_UM` (2 mm) in plane interiors (`ADAPTIVE_GUARD`
+  sets the clearance a block needs to grow). The **minimum element size
+  is the grid cell size itself** (auto / dialog / `CELL_UM_OVERRIDE`);
+  the uniform limit reproduces the normal grid exactly. Large
+  speed/memory wins on big pours; the coarse–fine interfaces carry a
+  first-order flux error that biases R **low by ~0.5–2 %** depending on
+  geometry (worst on narrow strips, mild on large planes). All fields
+  are expanded back to the fine grid for the maps and reports.
 
 ## Offline / development