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>

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