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>

sras_compute.py

@@ -35,15 +35,17 @@ try:
 except ImportError:
     threadpool_limits = None
 
-_fft_backend = "numpy"   # "numpy" (scipy.fft) or "pyfftw"; set via set_fft_backend()
+_fft_backend = "scipy"   # "scipy" or "pyfftw"; set via set_fft_backend()
 
 
 def set_fft_backend(name: str):
     """Select the rfft implementation. Module-level state, so it must be set
     explicitly inside each multiprocessing child — it does not survive a
-    spawn."""
+    spawn. "numpy" is accepted as a legacy alias for "scipy"."""
     global _fft_backend
-    _fft_backend = name if (name != "pyfftw" or PYFFTW_AVAILABLE) else "numpy"
+    if name == "numpy":
+        name = "scipy"
+    _fft_backend = name if (name == "pyfftw" and PYFFTW_AVAILABLE) else "scipy"
 
 
 def get_fft_backend() -> str:
@@ -599,13 +601,17 @@ def compute_rf_image(sras: SrasFile, angle_idx: int,
 # ---------------------------------------------------------------------------
 
 def cache_file(path: str, mode: str, apply_bg_sub: bool,
-               fft_backend: str = "numpy", max_workers: int = 0) -> str:
+               fft_backend: str = "scipy", max_workers: int = 0) -> str:
     """Compute and store DC or FFT images for every angle of one file,
     converting v6 → v7 in place. Returns "" on success or an error message.
 
     Module-level and picklable so it can run in a ProcessPoolExecutor. The
     FFT backend and worker cap are passed explicitly because module globals
     do not survive a spawn.
+
+    The stored FFT cache is always natural-resolution (pad 1): the v7 SFFT
+    block records no pad factor, and padded views compute live fast enough
+    (see _peak_bins_zoom) that caching them is not worth a format change.
     """
     global _MAX_WORKERS
     try:
@@ -621,17 +627,26 @@ def cache_file(path: str, mode: str, apply_bg_sub: bool,
                     "can be batch-cached")
 
         n = sras.n_angles
+        n_workers, angle_budget = plan_angle_level(sras)
         if mode == "dc":
-            dc3 = [adc_to_mv(compute_dc_image(sras, a, CH3_IDX), *sras.cal(CH3_IDX))
-                   for a in range(n)]
-            dc4 = [adc_to_mv(compute_dc_image(sras, a, CH4_IDX), *sras.cal(CH4_IDX))
-                   for a in range(n)]
+            dc3 = _parallel_map(
+                lambda a: adc_to_mv(
+                    compute_dc_image(sras, a, CH3_IDX, max_workers=1,
+                                     budget=angle_budget), *sras.cal(CH3_IDX)),
+                range(n), n_workers)
+            dc4 = _parallel_map(
+                lambda a: adc_to_mv(
+                    compute_dc_image(sras, a, CH4_IDX, max_workers=1,
+                                     budget=angle_budget), *sras.cal(CH4_IDX)),
+                range(n), n_workers)
             sras.write_v7_cache(new_dc3_mv=dc3, new_dc4_mv=dc4)
         else:
             effective_bg = apply_bg_sub and sras.background is not None
             # dc_threshold_mv=None: store unmasked images and mask at display
             # time (same convention as v5's PREC block). Skipping the mask
-            # also skips reading CH4 entirely.
+            # also skips reading CH4 entirely. The FFT path parallelises
+            # internally over blocks, so angles run one at a time with the
+            # full budget.
             freq = [compute_rf_image(sras, a, dc_threshold_mv=None,
                                      apply_bg_sub=effective_bg)
                     for a in range(n)]