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>

fill_resistance/config.py

@@ -55,6 +55,20 @@ ELECTRODE_POS_LAYER = "User.1"  # rectangles on this layer mark V+ contact parts
 ELECTRODE_NEG_LAYER = "User.2"  # rectangles on this layer mark V- contact parts
 ALWAYS_REFILL = False           # refill zones even if KiCad says they are filled
 
+# --- Adaptive grid ---
+ADAPTIVE_CELLS = False          # solve on a 2:1-balanced quadtree: fine at
+                                # copper boundaries/electrodes/features,
+                                # coarse plane interiors (dialog-toggleable).
+                                # Slight low bias (~0.5-1% on feature-dense
+                                # boards); large speed/memory wins on pours
+ADAPTIVE_MAX_CELL_UM = 2000.0   # coarsest leaf edge length. The MINIMUM
+                                # element size is the grid cell size itself
+                                # (auto / dialog / CELL_UM_OVERRIDE). Rarely
+                                # needs tuning: the guard distance already
+                                # caps leaf growth near features
+ADAPTIVE_GUARD = 4              # a leaf of size s needs >= GUARD*s cells of
+                                # clearance to the nearest feature
+
 # --- Solver ---
 CONTACT_MODEL = "uniform"       # "uniform": conductor pressed on top injects
                                 # orthogonally with uniform surface density