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>

sras_viewer/common.py

@@ -6,7 +6,7 @@ from PyQt6.QtWidgets import (
     QScrollArea, QSizePolicy, QVBoxLayout, QWidget,
 )
 
-from sras_format import CH1_IDX, CH3_IDX, CH4_IDX
+from sras_format import CH1_IDX, CH3_IDX, CH4_IDX, _axes_extent  # noqa: F401 (re-exported)
 
 # ---------------------------------------------------------------------------
 # Display constants
@@ -112,13 +112,6 @@ def _combo(items=(), *, min_chars: int = 10) -> QComboBox:
     return combo
 
 
-def _axes_extent(x_axis, y_axis, dx: float, dy: float) -> list[float]:
-    """Matplotlib imshow extent with half-pixel margins, Y flipped so row 0
-    renders at the top."""
-    return [x_axis[0] - dx / 2, x_axis[-1] + dx / 2,
-            y_axis[-1] + dy / 2, y_axis[0] - dy / 2]
-
-
 def _wrap_label(text: str = "", css: str | None = None) -> QLabel:
     """A word-wrapped QLabel that reports its *wrapped* height to the layout.