Add min peak frequency floor (CACH v4) and Batch Export View as Images

Two strands of in-progress work, committed together because they overlap
in sras_workers.py and main_window.py.

Min peak frequency floor:
- CACH tail bumped to version 4, adding u32 min_freq_khz provenance in
  fixed-point kHz (a float32 20.1 reads back as 20.10000038 and would
  report a spurious mismatch forever). v1-v3 tails read as no floor.
- Stored FFT caches are accepted when the reader's floor is at or above
  the stored one, since a higher floor is re-applicable by masking.
- Floor plumbed through compute_rf_image, BatchCacheWorker and the viewer.

Batch Export View as Images:
- New sras_render.py holds draw_view_image, shared by the Qt canvas and
  the headless exporter so a PNG cannot drift from what the GUI shows.
  Deliberately Qt-free so it is importable in a pool subprocess.
- BatchExportImagesWorker renders the current view settings across many
  files, process-pooled with an inline fallback, reporting per-file
  output names so the caller can flag same-stem collisions.
- _axes_extent extracted into sras_format for both render paths.

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

tests/test_gui.py

@@ -246,8 +246,16 @@ def test_highlight_masked_pixels(ctx):
         win._redraw_image(win._current_image)
     shown, bad_color = calls[-1]
     assert bad_color is not None, "highlight color set while checkbox is on"
-    assert np.array_equal(np.isnan(shown), expect_masked), \
-        "NaN exactly where DC4 is below threshold, nowhere else"
+    # The highlight masks by value too: in an FFT mode, exactly 0 is the
+    # "no valid peak" sentinel (DC-masked, below the min-freq floor, or an
+    # empty spectrum), so the NaN set is the union of the DC mask and the
+    # zero-valued pixels. On this fixture every above-threshold pixel has a
+    # nonzero peak, so the union equals the DC mask alone.
+    expect_nan = expect_masked | (win._current_image == 0)
+    assert np.array_equal(np.isnan(shown), expect_nan), \
+        "NaN where DC4 is below threshold or the value-0 sentinel, nowhere else"
+    assert np.array_equal(expect_nan, expect_masked), \
+        "fixture precondition: every valid pixel has a nonzero peak"
 
     win.chk_highlight_masked.setChecked(False)
     calls.clear()