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