Add manual angle alignment mode with overlay, keyboard nudge, and save/clear

Automatic alignment's phase-correlation translation search is unreliable, so
add a Fusion -> Manual Alignment dialog: every angle's CH4 threshold mask
overlaid at once with distinct colors/opacity, a selector for the active
angle, arrow keys to nudge translation and Q/E to nudge rotation, an Auto
De-rotate button that snaps to the known scan angle, and Save/Clear
controls backed by a JSON sidecar that's restored automatically on reload.

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

sras_workers.py

@@ -304,3 +304,45 @@ class AngleAlignmentWorker(QObject):
             self.finished.emit(result, "")
         except Exception as exc:
             self.finished.emit(None, str(exc))
+
+
+class Ch4MaskWorker(QObject):
+    """Fetches each requested angle's CH4 (Bias B) DC image in mV, for
+    ManualAlignmentDialog's initial threshold-mask overlay.
+
+    Reuses dc_image_mv, which prefers a stored v5/v7 cache over recomputing
+    from raw waveforms, so this only does real work for a file that hasn't
+    gone through the v7 "Convert" batch step and for angles the main
+    window's own DcPrecomputeWorker (which runs automatically right after
+    every file load) hasn't reached yet. In the common case — the user opens
+    Fusion -> Manual Alignment after DC precompute has already finished —
+    *angle_indices* is empty and this worker is never even constructed (see
+    ManualAlignmentDialog._start_mask_prep).
+    """
+    angle_done = pyqtSignal(int, np.ndarray)   # angle_idx, dc4_mv
+    finished = pyqtSignal()
+    error = pyqtSignal(str)
+
+    def __init__(self, sras: SrasFile, angle_indices: list[int]):
+        super().__init__()
+        self._sras = sras
+        self._angles = angle_indices
+
+    def run(self):
+        try:
+            n_workers, budget = compute.plan_angle_level(self._sras)
+            pool = ThreadPoolExecutor(max_workers=n_workers)
+            try:
+                futures = {
+                    pool.submit(dc_image_mv, self._sras, a, CH4_IDX,
+                               max_workers=1, budget=budget): a
+                    for a in self._angles
+                }
+                for fut in as_completed(futures):
+                    a = futures[fut]
+                    self.angle_done.emit(a, fut.result())
+            finally:
+                pool.shutdown(wait=True)
+            self.finished.emit()
+        except Exception as exc:
+            self.error.emit(str(exc))