Persist FFT settings, fix backend naming, parallelise batch DC caching

- FFT backend and pad factor persist across sessions via QSettings
  (IniFormat; tests redirect the settings path for hermeticity).
- The default backend was labelled "NumPy FFT" but always dispatched to
  scipy.fft — rename the canonical value to "scipy" ("numpy" stays as a
  legacy alias) and fix the dialog label.
- cache_file: DC caching fans out over angles via _parallel_map with
  per-angle budgets (the DcPrecomputeWorker pattern); FFT caching stays
  serial per angle because compute_rf_image now parallelises internally
  over blocks. Documented that the v7 FFT cache is natural-resolution
  (pad 1) by design.

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

tests/conftest.py

@@ -1,8 +1,16 @@
-"""Shared test setup: repo-root imports and the offscreen Qt platform."""
+"""Shared test setup: repo-root imports, the offscreen Qt platform, and
+hermetic QSettings (tests must not read or write the user's real viewer
+settings)."""
 
 import os
 import sys
+import tempfile
 from pathlib import Path
 
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from PyQt6.QtCore import QSettings  # noqa: E402
+
+QSettings.setPath(QSettings.Format.IniFormat, QSettings.Scope.UserScope,
+                  tempfile.mkdtemp(prefix="sras_qsettings_"))