Experimental: push |J| heatmap overlays into KiCad (dialog opt-in)

After a solve, the per-layer current-density maps can be pushed into
the open board as reference images on User.9..User.12 (stackup order,
top first) - visible right in the editor, toggled like any layer,
never plotted to gerbers. Dialog checkbox, default OFF; every push
replaces all reference images on those layers.

Rendering (fill_resistance/overlay.py, KiCad-free and tested headless):
one pixel per grid cell, opaque over copper with the log scale lifted
off the colormap's near-black bottom (dark canvas), transparent
elsewhere, one pixel of half-alpha edge bleed so the overlay reaches
the drawn outline instead of stopping half a cell short. Pushing lives
in board_io (ReferenceImage via the IPC API, KiCad >= 10.0.1; scale =
width / (pixels * 1 inch / 300 PPI), position = image center); kipy
0.7.1 swallows creation errors, so the per-item status is read from
the raw CreateItemsResponse. pipeline.run takes an optional overlay
callback; failures are reported, never fatal.

tools/kicad_overlay_test.py pushes a fiducial alignment pattern (KiCad
bbox readback verified placement to half a pixel); tools/
kicad_heatmap_overlay.py runs the whole thing headless against the
open board, filtering marker rectangles of other nets' analyses via
the exact copper test.

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

fill_resistance/dialog.py

@@ -39,6 +39,7 @@ class Selection:
     vias_capped: bool = True
     cap_max_drill_mm: float = 0.5
     adaptive: bool = True
+    push_overlays: bool = False   # EXPERIMENTAL in-KiCad |J| overlays
 
 
 class _Dialog(QDialog):
@@ -121,6 +122,14 @@ class _Dialog(QDialog):
         self.extracu_edit.setEnabled(bool(buildup_layers))
         form.addRow("Extra Cu in openings [µm]:", self.extracu_edit)
 
+        first, last = config.OVERLAY_LAYERS[0], config.OVERLAY_LAYERS[-1]
+        self.overlay_check = QCheckBox(
+            f"experimental: push per-layer |J| heatmaps into the board as "
+            f"reference images on {first}..{last} (replaces images there; "
+            f"layers must be enabled in Board Setup)")
+        self.overlay_check.setChecked(config.PUSH_OVERLAYS)
+        form.addRow("Overlays:", self.overlay_check)
+
         buttons = QDialogButtonBox(QDialogButtonBox.Ok | QDialogButtonBox.Cancel)
         buttons.accepted.connect(self._try_accept)
         buttons.rejected.connect(self.reject)
@@ -237,7 +246,8 @@ class _Dialog(QDialog):
                          include_tracks=self.tracks_check.isChecked(),
                          vias_capped=self.capped_check.isChecked(),
                          cap_max_drill_mm=cap_max_drill,
-                         adaptive=self.adaptive_check.isChecked())
+                         adaptive=self.adaptive_check.isChecked(),
+                         push_overlays=self.overlay_check.isChecked())
 
     def _try_accept(self) -> None:
         try: