Add Export Fused ROI CSV export

Lets a user export the current ROI as one CSV with a column per selected
angle's value (RF peak freq, Bias A, Bias B, or velocity), once angles share
a common (x, y) grid -- either via a live Fusion alignment result or because
the open file is itself a previous Alignment Wizard export whose angles
already share one grid on disk. Never triggers a background compute; angles
without cached/stored data for the chosen value type are simply unavailable
in the picker.

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

sras_viewer/__init__.py

@@ -23,5 +23,7 @@ from .canvases import (                                               # noqa: E4
     AlignOverlayCanvas, ImageCanvas, RoiQuad, WaveformCanvas,
 )
 from .common import CH_LABELS, CMAPS, VELOCITY_MODE_IDX              # noqa: E402,F401
-from .dialogs import FftOptionsDialog, RowAverageFftOptionsDialog    # noqa: E402,F401
+from .dialogs import (                                                # noqa: E402,F401
+    FftOptionsDialog, FusedRoiExportDialog, RowAverageFftOptionsDialog,
+)
 from .main_window import SrasViewerWindow, main                      # noqa: E402,F401