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>

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
 
@@ -14,8 +15,20 @@ 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
-    out.mkdir(parents=True, exist_ok=True)
+    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