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>

pyproject.toml

@@ -28,6 +28,7 @@ sras-viewer = "sras_viewer.main_window:main"
 py-modules = [
     "sras_format",
     "sras_compute",
+    "sras_render",
     "sras_workers",
     "sras_align_export",
     "sras_average",