Keep stored row-averaged caches eligible on the fallback compute path

ComputeWorker dropped row_avg_n, so a stored row-averaged cache was
ineligible whenever the fallback compute served the image instead of the
DC precompute path. Thread the viewer's precomputed_row_avg_n through
ComputeWorker and the batch export worker's compute_rf_image call.

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

sras_viewer/main_window.py

@@ -1291,6 +1291,10 @@ class SrasViewerWindow(QMainWindow):
             # need to re-read the CH4 channel from disk.
             dc4_mv=self._dc_cache.get((angle_idx, CH4_IDX)),
             is_fft_mode=is_fft,
+            # Mirror _stored_fft_image: a stored row-averaged cache must stay
+            # eligible even when this fallback compute is the one that ends
+            # up serving it (e.g. before DC precompute reaches this angle).
+            row_avg_n=self._sras.precomputed_row_avg_n,
         )
         if not self._run_worker(
                 Jobs.COMPUTE, worker,