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>

README.md

@@ -75,6 +75,17 @@ SWIG API. Requires KiCad **10.0.1+**.
    per-via current and dissipation, and the **current through each
    injection area** — computed flux with the equipotential model,
    prescribed area share with the uniform model), `geometry_dump.json`.
+5. **Experimental — overlays inside KiCad** (dialog checkbox, default
+   off; KiCad ≥ 10.0.1): after the solve, the per-layer **|J| heatmaps
+   are pushed into the open board** as unlocked reference images on
+   `User.9`…`User.12` (`OVERLAY_LAYERS`; enable them in Board Setup),
+   copper layers mapped in stackup order, top first. Toggle them in the
+   Appearance panel like any layer; opaque over copper, transparent
+   elsewhere, cold end lifted so it stays visible on the dark canvas.
+   Reference images never plot to gerbers. Every push **replaces all
+   reference images on those layers**, so don't store unrelated images
+   there. Also available headless:
+   `python tools/kicad_heatmap_overlay.py --net X --amps 10`.
 
 ## Model & limits