Model sub-resolution traces as exact 1D resistor chains

Tracks are now first-class Problem objects (TrackSeg: centerline +
width, dump schema v5), so the wide/narrow decision replays at raster
time: traces at least TRACK_1D_FACTOR (3) cells wide rasterize from
their outline as before; narrower ones mark the cells their centerline
crosses as copper and connect them with explicit conductance links
carrying the trace's TRUE arc length per link - no staircase inflation
for diagonals or arcs, and no discretization error in the trace R, at
any grid size. Links across cells already joined by pour faces are
skipped (union, not sum); chain-only cells get no sheet faces (their
copper is narrower than a cell). Electrodes, via barrels, connectivity
restriction and the skin-effect scaling all work on chain cells
unchanged.

This removes the need to shrink the cell size for thin traces: a 0.2 mm
bridge at 500 um cells now matches its finely-rasterized ground truth
within a few percent (tested), including diagonal and arc traces.

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

fill_resistance/standalone.py

@@ -61,6 +61,7 @@ def main(argv=None) -> int:
     if args.layers:
         keep = [s.strip() for s in args.layers.split(",")]
         problem.layers = [l for l in problem.layers if l.layer_name in keep]
+        problem.tracks = [t for t in problem.tracks if t.layer_name in keep]
         if not problem.layers:
             print(f"ERROR: no layer of the dump matches --layers {args.layers}",
                   file=sys.stderr)