Speed up rasterization ~40x and large solves ~2x

- Hybrid rasterizer: PIL scanline fill for the bulk, with cells in a
  ~2 px band around each ring edge re-tested exactly against the
  polygon - cell-for-cell identical to the old center-in-polygon pass
  (equivalence test added) but O(vertices + cells) instead of
  O(vertices x cells). Measured 4.5 s -> 0.11 s at 1.45M cells with
  8.8k polygon vertices.
- AMG-preconditioned CG (pyamg, new requirement) above 500k unknowns:
  measured 7.0 s vs 15.3 s spsolve at 1.4M unknowns at a fraction of
  the memory, R identical to 1e-6; the old Jacobi-CG (kept as fallback
  when pyamg is missing) needed tens of minutes there. spsolve stays
  the default below 500k where it is exact and fastest.

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

metadata.json

@@ -2,7 +2,7 @@
     "$schema": "https://go.kicad.org/pcm/schemas/v2",
     "name": "Fill Resistance",
     "description": "DC/AC resistance of copper zone fills between two contacts, single- or multi-layer with via coupling, with current and power density maps.",
-    "description_full": "Computes the DC or AC resistance of copper zone fills between two contacts (marker rectangles on User.1/User.2 and/or selected pads), single- or multi-layer: the chosen net's fills are solved as coupled finite-difference sheets linked by the net's via and through-hole-pad barrels.\n\nShows per-layer rasterized maps, potential, current density and power density, reports per-via currents (via ampacity) and total dissipation at a selectable test current. At a user-set frequency the exact 1D foil/barrel skin-effect correction is applied (AC results are a rigorous lower bound). PNGs, a text summary and a re-solvable geometry dump are saved per run.\n\nNote: the first load builds the plugin's Python environment (numpy, scipy, matplotlib, PySide6) and can take several minutes.",
+    "description_full": "Computes the DC or AC resistance of copper zone fills between two contacts (marker rectangles on User.1/User.2 and/or selected pads), single- or multi-layer: the chosen net's fills are solved as coupled finite-difference sheets linked by the net's via and through-hole-pad barrels.\n\nShows per-layer rasterized maps, potential, current density and power density, reports per-via currents (via ampacity) and total dissipation at a selectable test current. At a user-set frequency the exact 1D foil/barrel skin-effect correction is applied (AC results are a rigorous lower bound). PNGs, a text summary and a re-solvable geometry dump are saved per run.\n\nNote: the first load builds the plugin's Python environment (numpy, scipy, pyamg, matplotlib, PySide6) and can take several minutes.",
     "identifier": "th.co.b4l.fill-resistance",
     "type": "plugin",
     "author": {