Stop discarding precomputed FFT data on view switches

The display path hardcoded row_avg_n=0 when checking a file's stored FFT
cache, since the main window has no row-averaging control (only the
batch-only RowAverageFftOptionsDialog). Once a file was batch-computed with
row-averaging, every view switch saw a phantom provenance mismatch and
silently launched a full raw recompute, discarding the precomputed data.

The display now asks for the stored image at the settings it was actually
computed under (bg-sub/pad/row-averaging), rather than the window's live
controls, so a stored or already-computed image is always shown once
present. Those controls now only affect a first-time compute for an
uncached angle or an explicit batch recompute -- never what's already on
screen. DC threshold is unaffected: it stays live, since re-masking a
cached image is free.

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

tests/test_gui.py

@@ -222,16 +222,25 @@ def test_threshold_change_recomputes(ctx):
 
 
 def test_bg_sub_toggle(ctx):
+    """bg-sub no longer gates the display: it only affects a future live
+    compute for an angle with nothing cached yet, or an explicit batch
+    recompute. Toggling it on an angle that already has an FFT image must
+    leave that image on screen, untouched."""
     win = ctx.win
     n_before = len(win._fft_cache)
+    img_before = win._current_image
     win.chk_bg_sub.setChecked(False)
-    assert wait_until(
-        lambda: not win._job_running("compute") and len(win._fft_cache) > n_before), \
-        "recomputed without bg-sub"
-    win.chk_bg_sub.setChecked(True)
     pump(200)
     assert not win._job_running("compute"), \
-        "returning to bg-sub was a cache hit (no recompute)"
+        "toggling bg-sub alone must not dispatch a recompute"
+    assert len(win._fft_cache) == n_before, "no new cache entry from the toggle"
+    assert np.array_equal(win._current_image, img_before), \
+        "displayed image unchanged by the bg-sub toggle"
+    win.chk_bg_sub.setChecked(True)
+    pump(200)
+    assert not win._job_running("compute")
+    assert len(win._fft_cache) == n_before
+    assert np.array_equal(win._current_image, img_before)
 
 
 def test_roi_and_csv_export(ctx):