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/config.py

@@ -95,6 +95,25 @@ OVERLAY_ALPHA = 255             # overlay opacity over copper (0-255);
                                 # translucency washes out over bright
                                 # copper - toggle the User layer instead
 
+# --- Low-current copper marking (EXPERIMENTAL) ---
+TRIM_ENABLED = False            # dialog default: mark the copper below
+                                # TRIM_THRESHOLD_PCT as polygons on
+                                # TRIM_LAYERS. A suggestion, not a safe
+                                # cut list: copper carries little current
+                                # BECAUSE the rest carries it - removal
+                                # redistributes |J|, re-run after changes
+TRIM_THRESHOLD_PCT = 10.0       # threshold as % of the mean |J| over all
+                                # solved copper cells (mean, not max:
+                                # contact-corner spikes would dwarf a
+                                # max-relative threshold); dialog-settable
+TRIM_LAYERS = ("User.5", "User.6", "User.7", "User.8")
+                                # copper layers map here in stackup order
+                                # (top first); existing polygons on these
+                                # layers are REPLACED on every push; each
+                                # must be enabled in Board Setup
+TRIM_MIN_AREA_MM2 = 0.5         # marked specks smaller than this are
+                                # dropped (nothing useful to reclaim)
+
 # --- Adaptive grid ---
 ADAPTIVE_CELLS = True           # solve on a 2:1-balanced quadtree: fine at
                                 # copper boundaries/electrodes/features,