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/main.py

@@ -102,9 +102,14 @@ def main() -> None:
             raise UserFacingError(f"KiCad API error: {e}")
 
         report.write_geometry_dump(outdir, problem)
+        overlay_cb = None
+        if selection.push_overlays:
+            def overlay_cb(stack, result):
+                board_io.push_result_overlays(board, stack, result)
         pipeline.run(problem, outdir, show=True, i_test=selection.current_a,
                      freq_hz=selection.freq_hz,
-                     contact_model=selection.contact_model)
+                     contact_model=selection.contact_model,
+                     overlay=overlay_cb)
     except UserFacingError as e:
         _fail(str(e), outdir)
     except Exception: