Merge the Documents/sras-viewer working copy into this repo

The two clones had diverged from origin/main (191d1b8) and both had
grown uncommitted work. This merges the other clone's three commits and
reconciles four conflicting files.

Both clones independently grew a feature called "batch export", but they
are different sweeps and both are kept:
- BatchExportWorker (this clone) — every angle x channel of the one open
  file, fixed colorbar per channel, under a new Export menu.
- BatchExportImagesWorker (other clone) — the current view across many
  selected files, process-pooled, under Convert.

Conflict resolutions of note:
- sras_average.py: the other clone's memory-bounded rewrite had silently
  reverted this clone's float32 -> int16 accumulator fix, which exists to
  keep averaged output from landing 1 ADC count off the original tool.
  Kept the rewrite, re-applied the fix, and corrected the two docstrings
  that still claimed float32.
- tests/test_compute.py: kept the other clone's meta["waveforms"] key
  (meta["data"] no longer exists for this fixture and would KeyError)
  and its laser_freq_hz assertions, plus this clone's int16 expectation
  and its dropped subprocess returncode assertions.
- ComputeWorker: row_avg_n and min_freq_mhz were added independently on
  either side; both are now threaded through.
- compute_rf_image's `exact` parameter is gone, removed by the other
  clone's PyFFTW peak-search rewrite. It had no remaining callers.

Verified: 149 passed with a complete dependency set. The failures seen
with this clone's own .venv are a missing scikit-image, which predates
this merge and reproduces identically on the pre-merge commit.

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

tools/make_test_sras.py

@@ -264,10 +264,7 @@ def write_rotating(path: Path, n_angles: int = 5, samples_per_frame: int = 4,
 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:
-    """Write a v2/v3/v4 file: uniform geometry, one flat waveform block.
-
-    Used to exercise sras_average.py, which only handles the legacy formats.
-    """
+    """Write a v2/v3/v4 file: uniform geometry, one flat waveform block."""
     rng = np.random.default_rng(seed)
     n_ch, bps = 3, 1