Mark low-current copper as polygons on user layers (dialog opt-in)

New EXPERIMENTAL dialog option (default off): after the solve, copper
whose |J| is below a threshold (default 10% of the mean |J| over all
solved copper cells - mean, not max, since contact-corner spikes would
dwarf a max-relative threshold) is vectorized into filled graphic
polygons on TRIM_LAYERS (User.5..User.8, configurable), one polygon
per region so Edit > Convert can turn one into a rule area by hand.
Areas are printed and the polygons saved to low_current_copper.json.

The mask -> polygon step is the 0.5 contour of the binary field via
contourpy (already in every venv as matplotlib dependency), padded so
regions touching the raster edge close, simplified with
Douglas-Peucker at 0.4 cells: staircase bevels collapse, one-cell-wide
strips survive. Specks under TRIM_MIN_AREA_MM2 are dropped.

Explicitly a suggestion, not a safe cut list (docstring, dialog and
README all say so): copper carries little current BECAUSE the rest
carries it, so removal redistributes |J| - the constant-density
optimizer that iterates this to convergence is future work.

board_io: the create/delete-with-status-surfaced helpers are now
generic (_create_items_checked / _remove_items_checked) and shared
between reference-image overlays and trim polygons.

Tested on the dev stack (3.13) and the Python 3.9 mac-stack venv, 148
passed each; contourpy 1.3.x has identical API on both.

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

fill_resistance/main.py

@@ -136,10 +136,17 @@ def main() -> None:
         if selection.push_overlays:
             def overlay_cb(stack, result):
                 board_io.push_result_overlays(board, stack, result)
+        trim_cb = None
+        if selection.trim_enabled:
+            def trim_cb(tr):
+                board_io.push_trim_polygons(board, tr)
         pipeline.run(problem, outdir, show=True, i_test=selection.current_a,
                      freq_hz=selection.freq_hz,
                      contact_model=selection.contact_model,
-                     overlay=overlay_cb)
+                     overlay=overlay_cb,
+                     trim_pct=(selection.trim_pct if selection.trim_enabled
+                               else None),
+                     trim_push=trim_cb)
     except progress.Cancelled:
         print("cancelled")             # user's own doing: no error figure
     except UserFacingError as e: