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>

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
@@ -26,6 +26,10 @@ CH1_DERIVED_MODES = (CH1_IDX, VELOCITY_MODE_IDX)
 
 CMAPS = ["gray", "viridis", "plasma", "inferno", "hot", "jet", "RdBu_r", "seismic"]
 
+# Fill color for masked (below-DC-threshold) pixels when "Highlight masked
+# pixels" is on, chosen to stand out against every colormap in CMAPS above.
+_MASKED_HIGHLIGHT_COLOR = "magenta"
+
 # (mode_str, status-bar unit, colorbar label) per channel index
 _CHANNEL_DISPLAY = {
     CH1_IDX: ("RF", "Peak frequency (MHz)", "MHz"),
@@ -109,13 +113,6 @@ 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.