Implement row-averaged FFT feature for same-row SNR cleanup

Add row-averaged FFT feature with configurable window size (row_avg_n
parameter) for improved signal-to-noise ratio on noisy scans. Includes:

- Gaussian-weighted same-row neighbor averaging (never crosses rows)
- Masked/renormalized convolution handling for edge cases and masked samples
- Cache format v2 with row_avg_n tracking to prevent silent cache mismatches
- GUI dialog option for row-average window configuration
- Comprehensive tests validating kernel properties, background subtraction
  invariance, and cache dispatch

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

sras_viewer/__init__.py

@@ -20,5 +20,7 @@ faulthandler.enable()   # print a native stack trace on SIGSEGV/SIGABRT/etc.
 
 from .canvases import ImageCanvas, RoiQuad, WaveformCanvas           # noqa: E402,F401
 from .common import CH_LABELS, CMAPS, VELOCITY_MODE_IDX              # noqa: E402,F401
-from .dialogs import FftOptionsDialog, ManualAlignmentDialog         # noqa: E402,F401
+from .dialogs import (                                                # noqa: E402,F401
+    FftOptionsDialog, ManualAlignmentDialog, RowAverageFftOptionsDialog,
+)
 from .main_window import SrasViewerWindow, main                      # noqa: E402,F401