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/dialog.py

@@ -40,6 +40,8 @@ class Selection:
     cap_max_drill_mm: float = 0.5
     adaptive: bool = True
     push_overlays: bool = False   # EXPERIMENTAL in-KiCad |J| overlays
+    trim_enabled: bool = False    # EXPERIMENTAL low-current copper marking
+    trim_pct: float = 10.0        # threshold as % of the mean |J|
 
 
 class _Dialog(QDialog):
@@ -130,6 +132,19 @@ class _Dialog(QDialog):
         self.overlay_check.setChecked(config.PUSH_OVERLAYS)
         form.addRow("Overlays:", self.overlay_check)
 
+        tfirst, tlast = config.TRIM_LAYERS[0], config.TRIM_LAYERS[-1]
+        self.trim_check = QCheckBox(
+            f"experimental: mark copper below the threshold as polygons "
+            f"on {tfirst}..{tlast} (replaces polygons there; a suggestion "
+            f"only - removing copper shifts current elsewhere)")
+        self.trim_check.setChecked(config.TRIM_ENABLED)
+        form.addRow("Low-current copper:", self.trim_check)
+
+        self.trim_edit = QLineEdit(f"{config.TRIM_THRESHOLD_PCT:g}")
+        self.trim_edit.setEnabled(config.TRIM_ENABLED)
+        self.trim_check.toggled.connect(self.trim_edit.setEnabled)
+        form.addRow("Threshold [% of mean |J|]:", self.trim_edit)
+
         buttons = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)
         buttons.accepted.connect(self._try_accept)
         buttons.rejected.connect(self.reject)
@@ -227,6 +242,12 @@ class _Dialog(QDialog):
             extra_cu = number(self.extracu_edit, "Extra Cu")
             if extra_cu < 0:
                 raise ValueError("Extra Cu must be ≥ 0 µm.")
+        trim_pct = config.TRIM_THRESHOLD_PCT
+        if self.trim_check.isChecked():
+            trim_pct = number(self.trim_edit, "Trim threshold")
+            if not 0 < trim_pct < 100:
+                raise ValueError("Trim threshold must be between 0 and "
+                                 "100 (% of the mean |J|).")
         cap_max_drill = config.CAP_MAX_DRILL_MM
         if self.capped_check.isChecked():
             cap_max_drill = number(self.cap_drill_edit, "Capped up to drill")
@@ -251,7 +272,9 @@ class _Dialog(QDialog):
                          vias_capped=self.capped_check.isChecked(),
                          cap_max_drill_mm=cap_max_drill,
                          adaptive=self.adaptive_check.isChecked(),
-                         push_overlays=self.overlay_check.isChecked())
+                         push_overlays=self.overlay_check.isChecked(),
+                         trim_enabled=self.trim_check.isChecked(),
+                         trim_pct=trim_pct)
 
     def _try_accept(self) -> None:
         try: