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/pipeline.py

@@ -12,7 +12,9 @@ from .solver import Result
 
 def run(problem: Problem, outdir: Path | None, show: bool = True,
         i_test: float | None = None, freq_hz: float = 0.0,
-        contact_model: str | None = None) -> Result:
+        contact_model: str | None = None, overlay=None) -> Result:
+    """overlay: optional callback(stack, result) run after the solve
+    (EXPERIMENTAL in-KiCad overlays); its failures are non-fatal."""
     if i_test is None:
         i_test = config.TEST_CURRENT_A
     if i_test <= 0:
@@ -43,6 +45,12 @@ def run(problem: Problem, outdir: Path | None, show: bool = True,
         report.write_summary(outdir, problem, stack, result)
     print(report.result_line(result, problem, stack))
 
+    if overlay is not None:
+        try:
+            overlay(stack, result)
+        except Exception as e:
+            print(f"overlay push failed: {e}")
+
     figs = [
         (plots.fig_raster(stack, e1, e2, problem, result), "1_raster_map"),
         (plots.fig_potential(result, stack, e1, e2, problem), "2_potential"),