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/config.py

@@ -78,6 +78,20 @@ ELECTRODE_POS_LAYER = "User.1"  # rectangles on this layer mark V+ contact parts
 ELECTRODE_NEG_LAYER = "User.2"  # rectangles on this layer mark V- contact parts
 ALWAYS_REFILL = False           # refill zones even if KiCad says they are filled
 
+# --- In-KiCad result overlays (EXPERIMENTAL) ---
+PUSH_OVERLAYS = False           # after solving, push the per-layer |J|
+                                # heatmaps into the open board as unlocked
+                                # reference images (editor-only, never
+                                # plotted); dialog-toggleable
+OVERLAY_LAYERS = ("User.9", "User.10", "User.11", "User.12")
+                                # copper layers map here in stackup order
+                                # (top first); existing reference images on
+                                # these layers are REPLACED on every push;
+                                # each must be enabled in Board Setup
+OVERLAY_ALPHA = 255             # overlay opacity over copper (0-255);
+                                # translucency washes out over bright
+                                # copper - toggle the User layer instead
+
 # --- Adaptive grid ---
 ADAPTIVE_CELLS = True           # solve on a 2:1-balanced quadtree: fine at
                                 # copper boundaries/electrodes/features,