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>

sras_format.py

@@ -523,7 +523,13 @@ class SrasFile:
         final_freq = new_freq_mhz if new_freq_mhz is not None else self.precomputed_freq_mhz
         final_bg_sub = new_bg_sub if new_bg_sub is not None else self.precomputed_bg_sub
 
-        dc_entries = [a for a in range(self.n_angles) if final_dc3[a] is not None]
+        # dc3/dc4 are always populated together by every current caller, but
+        # guard the per-angle pairing explicitly rather than assume it: an
+        # angle present in only one of the two arrays would otherwise crash
+        # below on final_dc4[a].astype(...) (or silently store the wrong
+        # dc3/dc4 pairing for that angle).
+        dc_entries = [a for a in range(self.n_angles)
+                     if final_dc3[a] is not None and final_dc4[a] is not None]
         fft_entries = [a for a in range(self.n_angles) if final_freq[a] is not None]
 
         block_flags = ((CACH_FLAG_DC if dc_entries else 0)
@@ -552,13 +558,23 @@ class SrasFile:
             f.truncate()
             f.flush()
             os.fsync(f.fileno())
-            # Version-byte flip last: if the process dies before this point,
-            # the file is still readable as plain v6 (v6 parsing only
-            # bounds-checks per-angle offset+nbytes <= file_size, it never
-            # asserts exactly how many bytes follow the last angle) — so an
-            # interrupted write can never corrupt the file, only leave
-            # harmless trailing bytes that the next successful write
-            # overwrites via this same deterministic cache offset.
+            # Version-byte flip last: when converting a v6 source (self.version
+            # was 6 on entry), if the process dies before this point the file
+            # is still readable as plain v6 (v6 parsing only bounds-checks
+            # per-angle offset+nbytes <= file_size, it never asserts exactly
+            # how many bytes follow the last angle) — so an interrupted write
+            # can never corrupt the file, only leave harmless trailing bytes
+            # that the next successful write overwrites via this same
+            # deterministic cache offset.
+            #
+            # That guarantee does NOT extend to updating an already-v7 file:
+            # the version byte here is already 7 before this call, so a crash
+            # during the payload write above (before flush/fsync/truncate)
+            # can leave a cache tail that mixes a prefix of the new payload
+            # with a stale suffix of the old one, and this method has no
+            # protection against that case (no atomic rename — the tail is
+            # rewritten in place to avoid copying the, potentially huge,
+            # waveform data that precedes it).
             f.seek(4)
             f.write(struct.pack("B", 7))
             f.flush()