Add batch export of DC/RF/Velocity map images

Add Export -> Batch Export Images..., which renders and saves one PNG
per angle for whichever of CH1 (RF), CH3 (DC-A), CH4 (DC-B), and
Velocity the user selects, for the currently open file. Each channel
gets its own fixed min/max colorbar range, entered once in the dialog
and held constant across every exported angle, so the resulting images
are directly comparable to each other instead of each auto-scaling to
its own data (today's live-view default).

BatchExportDialog (sras_viewer.py) collects the output folder, file
prefix, and per-channel checkbox + range, seeded from whatever's
already cached (session DC/FFT caches, or the file's own v7 cache
blocks) so opening the dialog never triggers a fresh compute. Export
runs on a background thread via a new BatchExportWorker
(sras_workers.py), which computes each angle's channel image with the
existing dc_image_mv/compute_rf_image helpers (reusing one FFT per
angle for both RF and Velocity) and renders it with a headless
matplotlib Agg canvas.

Also includes several small correctness/robustness fixes that were
already staged in the working tree: an int16-vs-float32 accumulator
mismatch in sras_average.py's row averaging, a DC4-mask cache reuse and
atomic sidecar write in sras_compute.py, a dc3/dc4 pairing guard in
sras_format.py's v7 cache writer, and matching updates to the
tools/ test fixtures.

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

tools/make_test_sras.py

@@ -11,12 +11,16 @@ Usage:
 
 import argparse
 import struct
+import sys
 from pathlib import Path
 
 import numpy as np
 
-HDR_FMT_V6 = ">4sBHfffffffIdBB"
-GEO_FMT_V6 = ">ffIH"
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+# Packed straight from sras_format.py's own constants (not a hand-copy) so
+# this generator can never silently drift from what the real parser expects.
+from sras_format import GEO_FMT_V6, HDR_FMT as HDR_FMT_LEGACY, HDR_FMT_V6  # noqa: E402
 
 # Per-angle (n_rows, n_frames) — deliberately different per angle so ragged
 # geometry handling is actually exercised.
@@ -125,9 +129,6 @@ def write(path: Path, n_angles: int = 3, seed: int = 0,
     return meta
 
 
-HDR_FMT_LEGACY = ">4sBHHffffIIdBB"
-
-
 def write_legacy(path: Path, version: int = 4, n_angles: int = 2,
                  n_rows: int = 4, n_frames: int = 10,
                  samples_per_frame: int = 32, seed: int = 0) -> dict: