Fix the two macOS field-test failures: backend order, RO board dir

Second Mac run (KiCad demo board opened from the mounted installer
image) got past the dialog and died twice:

1. make_output_dir crashed with OSError 30: the demos volume is a
   read-only filesystem. Now falls back to a temp directory named
   after the board, with the path printed (the Messages panel showed
   it is actually read there).

2. The error figure - and every other figure - could never render:
   matplotlib refused with Cannot load backend TkAgg ... as qt is
   currently running. macOS bundled Python ships tkinter, so the
   old tk-first probe picked TkAgg while the PySide6 dialog and
   progress window had already made this a Qt process. Windows never
   saw it because KiCad Python there has no tkinter. Qt now comes
   first - PySide6 is a hard dependency, so it is always there.

Both covered by tests that fail on the old code: the dev env has both
toolkits installed, so the backend-order assertion is exercised for
real, and the RO fallback is simulated by denying mkdir under the
board dir. 140 passed on the dev stack and on the Python 3.9 +
numpy 2.0 / scipy 1.13 / matplotlib 3.9 / PySide6 6.10 stack that a
Mac venv resolves.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -123,7 +123,10 @@ notes* below.
    multi-layer pours at fine cell sizes may run for minutes (on our
    test setup a typical real-board run finishes in ≈ 8 s). Then read
    R / voltage drop / total power in the figure titles and status
-   bar. Outputs land in `<board dir>/fill_res_results/<timestamp>/`:
+   bar. Outputs land in `<board dir>/fill_res_results/<timestamp>/`
+   (if the board directory is not writable — e.g. a demo project opened
+   straight from the mounted installer image — a temp directory is used
+   instead and its path printed to the Messages panel):
    per-layer `1_raster_map` / `2_potential` / `3_current_density` /
    `4_power_density` PNGs, `summary.txt` (incl. the busiest vias with
    per-via current and dissipation, and the **current through each

fill_resistance/plots.py

@@ -1,8 +1,9 @@
 """Figures: per-layer rasterized maps, potential, current density, power
 density, and the error figure. PNGs are saved BEFORE any window opens.
 
-Backend: interactive if a GUI toolkit exists (tkinter, else Qt), else Agg
-with os.startfile on the saved PNGs so results are never silent.
+Backend: interactive if a GUI toolkit exists (Qt first, tkinter as a
+fallback), else Agg with the OS default viewer on the saved PNGs so
+results are never silent.
 """
 from __future__ import annotations
 
@@ -18,19 +19,23 @@ import numpy as np
 
 def _pick_backend():
     """matplotlib.use() is lazy and 'succeeds' for backends whose GUI
-    toolkit is missing (KiCad's Python has no tkinter), so probe the
-    toolkits explicitly."""
-    try:
-        import tkinter  # noqa: F401
-        return "TkAgg"
-    except Exception:
-        pass
+    toolkit is missing (KiCad's Windows Python has no tkinter), so probe
+    the toolkits explicitly. Qt MUST come first: PySide6 is a hard
+    dependency and the selection dialog / progress window put a Qt event
+    loop in this process, after which matplotlib refuses TkAgg
+    ("Cannot load backend 'TkAgg' ... as 'qt' is currently running") -
+    exactly what happened on macOS, whose bundled Python ships tkinter."""
     for qt in ("PySide6", "PyQt6", "PyQt5", "PySide2"):
         try:
             __import__(qt)
             return "QtAgg" if qt in ("PySide6", "PyQt6") else "Qt5Agg"
         except Exception:
             continue
+    try:
+        import tkinter  # noqa: F401
+        return "TkAgg"
+    except Exception:
+        pass
     return None
 
 

fill_resistance/report.py

@@ -1,6 +1,7 @@
 """Output directory, summary.txt, geometry dump, stdout one-liner."""
 from __future__ import annotations
 
+import tempfile
 from datetime import datetime
 from pathlib import Path
 
@@ -14,7 +15,19 @@ from .solver import Result
 
 def make_output_dir(board_dir: Path) -> Path:
     stamp = datetime.now().strftime("%Y%m%d-%H%M%S")
-    out = Path(board_dir) / config.OUTPUT_DIRNAME / stamp
+    board_dir = Path(board_dir)
+    out = board_dir / config.OUTPUT_DIRNAME / stamp
+    try:
+        out.mkdir(parents=True, exist_ok=True)
+    except OSError as e:
+        # The board can live somewhere unwritable - e.g. the demos
+        # folder on the mounted KiCad installer image (read-only, and
+        # how the first macOS field test was run). Results still have
+        # to land somewhere the figures/summary can be written.
+        out = (Path(tempfile.gettempdir()) / config.OUTPUT_DIRNAME
+               / f"{board_dir.name}-{stamp}")
+        print(f"board directory not writable ({e}); saving results to "
+              f"{out}")
         out.mkdir(parents=True, exist_ok=True)
     return out
 

tests/test_platform_fallbacks.py

@@ -0,0 +1,45 @@
+"""Regressions from the first macOS field test.
+
+Two failures that only a non-Windows KiCad could produce: the board
+directory was read-only (demo project opened straight from the mounted
+installer image), and matplotlib picked TkAgg - macOS' bundled Python
+ships tkinter, unlike KiCad's Windows Python - then refused to create
+any figure because the PySide6 dialog already had a Qt event loop in
+the process.
+"""
+import pathlib
+
+from fill_resistance import plots, report
+
+
+def test_backend_prefers_qt_over_tk():
+    # The dev environment has both toolkits installed, so this asserts
+    # the preference order for real: Qt must win, because the selection
+    # dialog / progress window make the process a Qt process before the
+    # first figure exists.
+    assert plots._pick_backend() in ("QtAgg", "Qt5Agg")
+
+
+def test_output_dir_falls_back_when_board_dir_unwritable(
+        tmp_path, monkeypatch, capsys):
+    board_dir = tmp_path / "board"
+    board_dir.mkdir()
+    real_mkdir = pathlib.Path.mkdir
+
+    def deny_under_board(self, *args, **kwargs):
+        if str(self).startswith(str(board_dir)):
+            raise OSError(30, "Read-only file system", str(self))
+        return real_mkdir(self, *args, **kwargs)
+
+    monkeypatch.setattr(pathlib.Path, "mkdir", deny_under_board)
+    out = report.make_output_dir(board_dir)
+    assert out.is_dir()
+    assert not str(out).startswith(str(board_dir))
+    assert board_dir.name in out.name      # traceable back to the board
+    assert "not writable" in capsys.readouterr().out
+
+
+def test_output_dir_normal_case_unchanged(tmp_path):
+    out = report.make_output_dir(tmp_path)
+    assert out.is_dir()
+    assert out.parent.parent == tmp_path