Make both batch image exports reproduce the view on screen

Two strands, committed together because they touch the same four files
and answer the same question: an exported PNG must be the image the user
was looking at when they triggered the export.

Dropped view settings (bug fix):
- BatchExportWorker forwarded the DC threshold, bg-sub, pad and row-
  averaging to compute_rf_image but never min_freq_mhz, so every RF and
  Velocity map was exported with no min peak frequency floor. The pixels
  the floor exists to reject came back at their pre-floor peaks.
- That also decided *which source* the image came from: requesting floor
  0 makes a stored v7 cache look like a match (cache_mismatch_reasons
  only rejects a higher stored floor), so a batch-computed file exported
  the cached peak-frequency block verbatim - numbers from an earlier
  Batch Compute run, while the screen showed that same cache re-masked
  against the live floor. The symptom was a newly loaded file exporting
  a stale-looking velocity map, since the floor is a display control
  that survives a load while the window's own FFT cache does not.
- export_view_image had the same class of defect with a different
  parameter: it dropped row_avg_n, so a file whose stored cache is row-
  averaged displayed from the store but exported as a full raw
  recompute - a different, and much slower, image. It now reads
  sras.precomputed_row_avg_n per file, mirroring _stored_fft_image and
  _start_compute, and derived per file for the same reason n_fft is.

Export at the live canvas size:
- Both exporters now take the canvas's current figure size instead of a
  hard-coded 7x5. draw_view_image uses aspect="auto", so the figure box
  is what sets the map's proportions - a view sized wide on screen was
  being squeezed into a different shape on disk. Read at trigger time,
  in inches, so a resize mid-batch cannot change images later in the
  same run and a HiDPI display exports like a standard one.
- sanitize_figsize clamps a degenerate size (collapsed splitter pane,
  minimized window): a wrong-looking aspect ratio must never be the
  reason a batch loses an image.

Tests: BatchExportWorker had no coverage at all. Four new tests cover
the floor being applied, a stored cache never being served unfloored,
the menu-to-worker wiring, and the row-averaged cache case; each was
verified to fail against the unfixed code. 166 passed.

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

sras_viewer/main_window.py

@@ -1716,6 +1716,25 @@ class SrasViewerWindow(QMainWindow):
     # Batch export view as images
     # ------------------------------------------------------------------
 
+    def _view_figsize(self) -> tuple[float, float]:
+        """The image canvas's current figure size in inches — handed to every
+        image export so the PNG has the shape the view is being read at.
+
+        The canvas draws with aspect="auto" and expands with the window, so
+        the map's proportions are the widget's, not the data's; a renderer
+        with its own hard-coded size necessarily distorts the export
+        relative to the screen.
+
+        Inches rather than pixels because that is what Figure takes, and
+        matplotlib's Qt canvas keeps size_inches in logical units (it scales
+        figure.dpi by the device pixel ratio and divides physical pixels by
+        it), so the same window gives the same export on a HiDPI display as
+        on a standard one. Read at trigger time: a resize mid-batch must not
+        change the shape of images later in the same run.
+        """
+        w, h = self.image_canvas.figure.get_size_inches()
+        return (float(w), float(h))
+
     def _on_batch_export_images(self):
         if self._job_running(Jobs.BATCH):
             return
@@ -1751,6 +1770,7 @@ class SrasViewerWindow(QMainWindow):
             highlight_masked=self.chk_highlight_masked.isChecked(),
             mode_str=mode_str, colorbar_label=colorbar_label,
             mask_color=_MASKED_HIGHLIGHT_COLOR,
+            figsize=self._view_figsize(),
         )
         started = self._run_worker(
             Jobs.BATCH, worker,
@@ -1858,7 +1878,12 @@ class SrasViewerWindow(QMainWindow):
             cmap=self.combo_cmap.currentText(),
             apply_bg_sub=self.chk_bg_sub.isChecked(),
             dc_threshold_mv=self.spin_threshold_mv.value(),
-            n_fft=self._current_n_fft(), grating_um=self.spin_grating_um.value())
+            n_fft=self._current_n_fft(), grating_um=self.spin_grating_um.value(),
+            # Live, like every other display control here and like
+            # _stored_fft_image: the exported RF/Velocity map has to be the
+            # one on screen, floor included.
+            min_freq_mhz=self.spin_min_freq_mhz.value(),
+            figsize=self._view_figsize())
         started = self._run_worker(
             Jobs.EXPORT, worker,
             connect=(