Overlay push: verify deletes, clear unwritten slots, one undo step

kipy's Board.remove_items discards the DeleteItemsResponse, and the
proto warns the overall status may read OK even when nothing was
deleted - a locked image comes back IDS_IMMUTABLE. remove_overlays
went through the proto layer already for create; do the same for
delete, so a locked overlay is reported instead of silently surviving
while a second image is stacked on top of it.

Slots that a narrower run does not write kept the previous solve's
heatmap and read as current; clear them. The whole push is now one
commit, so a single undo reverts it rather than one layer of it.

_pad_polygons probed F.Cu before B.Cu regardless of which side the
joint protrudes from, so a pad sized differently per copper layer had
its solder coat measured from the wrong face; probe the solder side
first. Pads on no single copper layer are now noted rather than
silently skipped.

_fail could report nothing at all: before the output directory exists
no PNG is written, and with no GUI toolkit plots only opens saved
PNGs - the broken-plugin-environment case the docstring promises to
cover. Fall back to the temp dir, and never let reporting mask the
fault. Frequency input now keeps its specific rejection reason, as
the other numeric fields do.

fill_resistance/main.py

@@ -18,9 +18,21 @@ from .errors import CandidateError, UserFacingError
 
 def _fail(message: str, outdir) -> None:
     print(f"ERROR: {message}")
-    from . import plots
-    fig = plots.fig_error(message)
-    plots.save_and_show([(fig, "error")], outdir)
+    try:
+        if outdir is None:
+            # A failure before the run has an output directory (a broken
+            # plugin environment throws on import) would otherwise save
+            # no PNG - and with no GUI toolkit, plots falls back to
+            # opening the saved PNGs, so the figure would never be shown
+            # either. Exactly the case the docstring promises to cover.
+            import tempfile
+            from pathlib import Path
+            outdir = Path(tempfile.gettempdir()) / "fill-resistance-error"
+        from . import plots
+        fig = plots.fig_error(message)
+        plots.save_and_show([(fig, "error")], outdir)
+    except Exception:                     # reporting must not mask the fault
+        traceback.print_exc()
     sys.exit(1)