Phase 3: viewer reads v6 via mmap; extract analysis core

The viewer could only parse v2-v4 headers while the app has been writing
v6 for some time — it could not open ANY file the current app produces.
It now uses core.sras_format directly (v6 only, per user decision).

New core/sras_analysis.py (Qt-free): ChannelCalibration, image reducers,
and SawPipeline. sras_viewer.py keeps only Qt.

Memory (measured, 92 MB synthetic scan, separate processes):
  old eager path  +305 MB   read()+slice-copy+astype+float32 mean
  new mmap path   + 31 MB   zero-copy view + mean(dtype=)
  -> identical DC image; old scaled at ~3.3x file size, new at image size
- load_angle() returns a read-only mmap view instead of reading the whole
  data block, then copying it twice
- SAW sweeps keep one scalar per pixel (process_shot_metrics) instead of
  retaining 5 full arrays x pixel count in a results list
- CH1 float32 materializes only for pixels passing the DC mask
- matched filter caches the template FFT instead of recomputing per pixel
- opening a new file drops every reference to the old one (compute/
  template/diagnostic workers used to pin the previous multi-GB mapping)

Responsiveness:
- 250 ms debounce coalesces spinbox storms into one recompute
- grating change is a display-time scalar multiply, not a full FFT rerun
- colormap/clim reuse the AxesImage (set_data/set_clim) instead of
  clf() + rebuilding the colorbar; draw_idle() throughout
- SAW diagnostics (21 pipeline runs) and CSV export moved off the GUI thread

Also: ragged per-angle geometry is respected (v6 angles differ in rows/
frames), truncated scans show only rows present on disk, dead decimation
path and v2 fallback branch removed, scipy added to viewer requirements.

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

sras_viewer_requirements.txt

@@ -1,3 +1,4 @@
 PyQt6==6.10.2
 numpy==2.4.1
 matplotlib==3.10.8
+scipy==1.16.3